t/runtests: rename a variable

Signed-off-by: Niels Thykier <niels@thykier.net>
t/runtests: Add summary of skipped and failed tests

After all tests have been run, print a summary of skipped and failed
tests.  In the summary of skipped tests, include the reason for why
the test was skipped.

Signed-off-by: Niels Thykier <niels@thykier.net>