tests: Don't recurse into sys/compat32

Stubbing out the whole file means it has no targets, so recursive makes
fail with "don't know how to make X". We could fix that, but because the
level above is adding it to TESTS_SUBDIRS it'll generate an include line
for the subdirectory in its Kyuafile, which won't work (the problem that
was previously seen on non-aarch64 in CI). Thus we really need to not
even add it to TESTS_SUBDIRS in the first place.

Reported by: Dan Mack <mack@macktronics.com>
Fixes: 28f66935d400 ("tests: Disable sys/compat32 and sys/compat32/aarch64 tests")