Reset environment variables between builds.

I was seeing a problem where `MOCHA_RUN_INTEGRATION_TESTS` was being set in one
build but not set in a subsequent build. The value from the first build was
trumping the lack of one in the subsequent one.

It might be simpler to prefix commands with the environment variables to avoid
so much "global state", but this will do for now.
Force use of Test::Unit runner, etc in relevant integration tests

Prior to this, I don't think we were really testing the Mocha integration with
Test::Unit much, because, although `TestUnitTest` was a subclass of
`Test::Unit::TestCase`, the important test case instances are the temporary
ones built by `TestRunner#run_as_test` et al. Prior to this change, these would
only have used Test::Unit where MiniTest was not available *at all* i.e. only
in early versions of Ruby and when the MiniTest gem was not loaded.

I have intentionally not fixed the indentation in `test/test_runner.rb` so that
the diff is clearer. I intend to fix the indentation in a separate commit.
Fix indentation in `test/test_runner.rb`.
Test::Unit is not available in Ruby v1.9.3 standard library.

This only came to light after the fix in [1].

[1] 7fc0145bcdc6e96d261abd787968940cf7f8d068