Re: [PATCH v2 3/3] t/Makefile: get UNIT_TESTS list from C sources

> We decide on the set of unit tests to run by asking make to expand the
> wildcard "t/unit-tests/bin/*". One unfortunate outcome of this is that
> we'll run anything in that directory, even if it is leftover cruft from
> a previous build. This isn't _quite_ as bad as it sounds, since in
> theory the unit tests executables are self-contained (so if they passed
> before, they'll pass even though they now have nothing to do with the
> checked out version of Git). But at the very least it's wasteful, and if
> they _do_ fail it can be quite confusing to understand why they are
> being run at all.
> 
> This wildcarding presumably came from our handling of the regular
> shell-script tests, which use $(wildcard t[0-9][0-9][0-9][0-9]-*.sh).
> But the difference there is that those are actual tracked files. So if
> you checkout a different commit, they'll go away. Whereas the contents
> of unit-tests/bin are ignored (so not only do they stick around, but you
> are not even warned of the stale files via "git status").
> 
> This patch fixes the situation by looking for the actual unit-test
> source files and then massaging those names into the final executable
> names. This has two additional benefits:
> 
>    1. It will notice if we failed to build one or more unit-tests for
>       some reason (wheras the current code just runs whatever made it to
>       the bin/ directory).

>    2. The wildcard should avoid other build cruft, like the pdb files we
>       worked around in 0df903d402 (unit-tests: do not mistake `.pdb`
>       files for being executable, 2023-09-25).
> 
> Our new wildcard does make an assumption that unit tests are built from
> C sources. It would be a bit cleaner if we consulted UNIT_TEST_PROGRAMS
> from the top-level Makefile. But doing so is tricky unless we reorganize
> that Makefile to split the source file lists into include-able subfiles.
> That might be worth doing in general, but in the meantime, the
> assumptions made by the wildcard here seems reasonable.