Add -betterC testsuite for Phobos

[wilzbach]

Split of #5952 that only contains the testsuite (and a first minimal test) as #5952 is currently blocked (it tries to fix Tuple for betterC and there are still some unresolved issues with toHash).

This PR should be non-controversial and thus us allow to finally get started with a betterC testsuite.

make -f posix.mak betterC

[dlang-bot]

Thanks for your pull request, @wilzbach!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#6640"

[PetarKirov]

I think we should also have baremetal tests for things that don't depend even on libc. Then hopefully it would be easy to build them with ldc for WebAssembly. @kinke what do you think? @wilzbach also do you think we could build them on buildkite with ldc?

[wilzbach]

I think we should also have baremetal tests for things that don't depend even on libc.

Are you thinking about sth. like #6641?

@wilzbach also do you think we could build them on buildkite with ldc?

Yes, that should be possible (though this will might require approval from @andralex).
However, before I set up more CIs jobs, it's probably a good idea to figure out how such a testsuite would look like.