Centralize on tests/TestRunner/

Apple’s internal CI uses a different test harness implementation than the one in tests/TestRunner. This leaves certain aspects of testing subtly different between CI systems. The additional features should be ported over, and everyone should standardize on tests/TestHarness as the way to script automated running of FDB tests.

Compare the test harness implementations used by Apple’s internal CI and tests/TestRunner, then review the reference to tests/TestHarness in the issue body to resolve which path is intended. Identify the additional internal-CI features to port and verify that automated FoundationDB test scripts use the standardized harness consistently.

Written by the indexing model from the issue text.