Tests for bin/slimrb

Here are some tests as promised in #573.

The tests in `test/core/test_commands.rb` cover the basic functionality someone would expect, when studying the output of `slim --help`.

I went a bit mental and tested whether behaviour of both `slim --stdin` and `slim file` + output to stdout and file is the same in all cases (with some exceptions) - that's what `TestCommands#prepare_common_test` is for (the helper methods are documented for future reference).