Fixed path tests when running on Windows: removed drive prefixes from check

Fixes for #1280.

This gets the amount of errors down to 3 or 5 on my machine:
```
==> kernel (exunit)
................................................................................................................................................................
................................................................................................................................................................
................................................................................................................................................................
..................................................F................................................................................** (TokenMissingError) nofile
:1: missing terminator: ] (for "[" starting at line 1)
......................................** (throw) 1
............................................** (SyntaxError) nofile:1: unexpected token: end
.    :erlang.system_info({:purify,'Node: nonode@nohost'})
    init.erl:757: :init.do_boot/3

.........** (ErlangError) erlang error: 1
......    erl_eval.erl:569: :erl_eval.do_apply/6
    src/elixir.erl:152: :elixir.eval_forms/3
    c:/dev/elixir/elixir/lib/elixir/lib/code.ex:113: Code.do_eval_string/3
    c:/dev/elixir/elixir/lib/elixir/lib/kernel/cli.ex:264: Kernel.CLI.process_command/2
    c:/dev/elixir/elixir/lib/elixir/lib/enum.ex:604: Enum."-map/2-lc$^0/1-0-"/2
    c:/dev/elixir/elixir/lib/elixir/lib/kernel/cli.ex:18: Kernel.CLI."-main/1-fun-2-"/1
    c:/dev/elixir/elixir/lib/elixir/lib/kernel/cli.ex:39: Kernel.CLI.run/2
    init.erl:1054: :init.start_it/1

................................................................................................................................................................
...............................................................................................................** (SyntaxError) nofile:1: invalid token: \x{3042
}
.    :erlang.system_info({:purify,'Node: nonode@nohost'})
    init.erl:757: :init.do_boot/3

...** (SyntaxError) nofile:1: invalid token: µ
    :erlang.system_info({:purify,'Node: nonode@nohost'})
    init.erl:757: :init.do_boot/3

.F...........F...............................................................................................

Failures:

  1) test_argv (SystemTest)
     ** (SyntaxError) nofile:1: syntax error before: 'ERROR'
     stacktrace:

  2) test_possible_deadlock (Kernel.CLI.CompileTest)
     ** (ExUnit.AssertionError) expected foo.ex to miss module Bar
     at test/elixir/kernel/cli_test.exs:88

  3) test_rm_rf_with_symlink (FileTest.Rm)
     ** (ExUnit.AssertionError) Expected false to be true
     at test/elixir/file_test.exs:774

Finished in 8.3 seconds (5.2s on load, 3.1s on tests)
1093 tests, 3 failures
make: *** [test_kernel] Error 1

Tom@Jommeke /c/dev/elixir/elixir
$ make test
==> elixir (compile)
==> elixir (eunit)
  All 213 tests passed.

==> kernel (exunit)
................................................................................................................................................................
................................................................................................................................................................
................................................................................................................................................................
..............................................................................F................................** (TokenMissingError) nofile:1: missing terminat
or: ] (for "[" starting at line 1)
.    :erlang.system_info({:purify,'Node: nonode@nohost'})
    init.erl:757: :init.do_boot/3

........................F.** (throw) 1
......................................................................** (SyntaxError) nofile:1: unexpected token: end
.    :erlang.system_info({:purify,'Node: nonode@nohost'})
    init.erl:757: :init.do_boot/3

.........** (ErlangError) erlang error: 1
...................................................................................................................................F............................
..................................................................................................** (SyntaxError) nofile:1: invalid token: \x{3042}
..........................F..** (SyntaxError) nofile:1: invalid token: µ
    :erlang.system_info({:purify,'Node: nonode@nohost'})
    init.erl:757: :init.do_boot/3

............F...............................................................................................

Failures:

  1) test_argv (SystemTest)
     ** (SyntaxError) nofile:1: syntax error before: 'ERROR'
     stacktrace:

  2) test_at_exit (SystemTest)
     ** (ExUnit.ExpectationError)
                  expected: []
       to be equal to (==): '0\n'
     at test/elixir/system_test.exs:51

  3) test_at_exit (Kernel.CLI.AtExitTest)
     ** (ExUnit.ExpectationError)
                  expected: 'goodbye '
       to be equal to (==): 'goodbye cruel world with status 0\n'
     at test/elixir/kernel/cli_test.exs:39

  4) test_possible_deadlock (Kernel.CLI.CompileTest)
     ** (ExUnit.AssertionError) expected foo.ex to miss module Bar
     at test/elixir/kernel/cli_test.exs:88

  5) test_rm_rf_with_symlink (FileTest.Rm)
     ** (ExUnit.AssertionError) Expected false to be true
     at test/elixir/file_test.exs:774

Finished in 8.4 seconds (5.1s on load, 3.2s on tests)
1093 tests, 5 failures
make: *** [test_kernel] Error 1
```

Apparently the `test_at_exit` code does not work all the time...