#580 Check overall test suite status after all test suites complete.

[Joseph Koshy]

Currently, make run-tests fails on the first test suite failure encountered, because test-suite invocation and directory traversal happen at the same time.

It may be more informative to run all the test suites and to then report the overall pass/fail status.

We cannot just use make -k run-tests, since we need the overall run to fail if any of the test suites had failed.