test suite: don't consider skipped individual tests as failing

It isn't completely clear what we want to do here, but

1) We currently don't fail if we skip a whole test file (mainly because
we neglect to count those skipped tests properly). This change at least
makes the two kinds of skipping consistent.

2) Automated build environments may have good reasons for building with
a minimal set of prereqs, and we don't want to discourage running our
test suite by breaking builds.