Fix Issue 21330 - __traits(getUnitTests) should include unittests fro…

[BorisCarvajal]

…m anonymous mixins

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21330	normal	__traits(getUnitTests) should include unittests from anonymous mixins

Testing this PR locally

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

dub run digger -- build "stable + dmd#11888"

[Geod24]

LGTM. However the behavior of symbolDg is error-prone, as it doesn't detect when a ScopeDsymbol is used. For example, unittest inside of extern(C++, ns) will be skipped, too.

[BorisCarvajal]

Similar to allMembers behavior.

[Geod24]

Just for clarity, I didn't mean to put the onus on you. This PR is perfect for stable, as it's self contained and fix the problem. It was more of a note for later.