[dmd-internals] Out of source builds for DMD testsuite

[David Nadlinger]

Hi all,

To properly integrate with the LDC build system, I adapted the DMD
testsuite so it can be run with an arbitrary result directory instead
of having test_results/ hardcoded.

Besides the build system integration aspect, this is useful if you
don't want to clutter the source directory, and also allows you to
have several instances of the test suite running in parallel (e.g. for
testing both 32 and 64 bit configurations).

I recently updated my pull request so that it works on the Windows
auto-tester machines as well:

https://github.com/D-Programming-Language/dmd/pull/1152

I'd appreciate quick reviews, as this tends to break all the time when
new test cases depending on test_results are added.

David