[stable] CI: Add extra Phobos tests (publictests and betterc make targets)

[kinke]

Adopting https://github.com/dlang/phobos/blob/9e7d2f37cfd36f42b7f2376ad07814297e40809f/.circleci/run.sh#L122-L133 to catch any compiler-caused regressions here for DMD CI, instead of breaking Phobos CI after the DMD merge.

After merging this, the Phobos GitHub Actions workflow will newly include these tests as well.

[dlang-bot]

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Testing this PR locally

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[kinke]

@dkorpel: This shows that #15343 (totally unexpectedly) broke Phobos CI, which runs some extra tests (in some CI jobs only, and on Linux only), that weren't tested here for DMD, and only those publictests now fail with undefined lambda symbols. Reverting the mangle/identifier change makes them pass again.

I've seen similar undefined-lambda-symbol regressions for a Symmetry codebase with LDC v1.40.0-beta5, which includes that mangling change from DMD stable. Beta4 was fine and there weren't any huge other changes that are likely to have caused those regressions.