[Issue 18235] Group logically similar tests into the same module in the D2 testsuite

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18235

--- Comment #6 from Iain Buclaw <ibuclaw at gdcproject.org> ---
Just be sure to benchmark, benchmark, and benchmark some more!

Things that have changed since the original raising of this include:

- Running the most expensive tests first had reduced the time it took to
complete by about a minute when using parallelism.
- Fail compilation tests no longer run with all permutations.
- Neither do compilable tests.

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