tests/shell: exit 77 from "run-tests.sh" if all tests were skipped

tests/shell: exit 77 from "run-tests.sh" if all tests were skipped

If there are multiple tests and some of them pass and some are skipped,
the overall result should be success (zero). Because likely the user
just selected a bunch of tests (or all of them). So skipping some tests
does not mean that the entire run is not a success.

However, if all tests are skipped, then mark the overall result as
skipped too. The more common case is if you only run one single test,
then we want to know, that the test didn't run.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>