ci: handle TEST_OUTPUT_DIRECTORY when printing test failures

ci: handle TEST_OUTPUT_DIRECTORY when printing test failures

The TEST_OUTPUT_DIRECTORY environment variable can be used to instruct
the test suite to write test data and test results into a different
location than into "t/". The "ci/print-test-failures.sh" script does not
know to handle this environment variable though, which means that it
will search for test results in the wrong location if it was set.

Update the script to handle TEST_OUTPUT_DIRECTORY so that we can start
to set it in our CI.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>