Explicitly call _d_assert_fail in test/exceptions/assert_fail.d

[MoonlightSentinel]

This test should only check the druntime component of -checkaction=context,
i.e. that _d_assert_fail yields an appropriate assert message. Using assert
directly linked this test to the lowering details and made changes changes
quite difficult because one cannot make changes to dmd and druntime in a
single PR (see e.g. dlang/dmd#11005).

CC @kinke, @wilzbach

[dlang-bot]

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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 run digger -- build "stable + druntime#3019"

[kinke]

Thanks, that will definitely help; at some point, these tests can be turned to regular unittests (for miniformat etc. too). I'd need it in stable though.

[MoonlightSentinel]

Rebased onto stable

[MoonlightSentinel]

BuildKite fails because it uses dmd-master ...