run_subtests() and run_subtests_with_signals() now treat a subtest exit
code of 77 as skipped rather than a failure, track skipped subtests
separately, and list them in the summary. Previously a subtest could
only fake a skip by exiting 0, which was indistinguishable from a real
pass.
Signed-off-by: Paul Meyer <katexochen0@gmail.com>