[test] Exit with an error if no tests are run.

If the test suite is misconfigured when it's run (a bad regexp, wrong test directory, etc.), the test suite may not discover any tests. When this happens, the test runner exits happily because no tests failed:

```
Ran 0 tests in 0.000s
RESULT: PASSED (0 passes, 0 failures, 0 errors, 0 skipped, 0 expected failures, 0 unexpected successes)
```

Change this to return an error so the misconfiguration can be more easily detected. Verified that `lldb-dotest -p TestDoesNotExist.py` successfully fails.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D85169