test-lib: show missing prereq summary

test-lib: show missing prereq summary

When running the full test suite many tests can be skipped because of
missing prerequisites. It not easy right now to get an overview of which
ones are missing.
When switching to a new machine or environment some libraries and tools
might be missing or maybe a dependency broke completely. In this case
the tests would indicate nothing since all dependant tests are simply
skipped. This could hide broken behaviour or missing features in the
build. Therefore this patch summarizes the missing prereqs at the end of
the test run making it easier to spot such cases.

 - Add failed prereqs to the test results.
 - Aggregate and then show them with the totals.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>