[PATCH 2/2] 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 test 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 match "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 build 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.

Signed-off-by: Jeff King <peff@peff.net>
---
I of course hit this when moving between "next" and "master" for an
up-and-coming unit-test file which sometimes failed.

 t/Makefile | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)