Re-enable unittest-debug Makefile to increase Phobos overall coverage

[wilzbach]

Follow-up to #5579

I will have a look if coverSetMerge can help here to get the best from both worlds.

It's already enabled, so running the unittest-debug as an extra step might increase the build time a bit, but should increase the overall coverage a bit.
(this is an experiment)

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[wilzbach]

Seems like it isn't that easy:

Never mind then, sorry.

[wilzbach]

Seems like it isn't that easy:

I forgot that we need to set coverage to merging, so I am testing whether a hack to call the respective Druntime function would improve things.

[wilzbach]

Still doesn't seem to work, I guess a good intermediate solution would be adding a -covmerge flag to DMD which simplifies this runtime fiddling for the users and Phobos.