I've set up an Xcode scheme for the WebKitTestRunner executable to run it, passing the full path to the test. The test uses waitUntilDone/notifyDone(), but WTR's watchdog fires immediately so the test always ends prematurely. In TestInvocation, m_timeout is always zero, even if --timeout is passed.
TestInvocation::initializeWaitToDumpWatchdogTimerIfNeeded() is always starting the watchdog timer with zero seconds.
<rdar://problem/50697654>
This is by design. Pass --no-timeout to get around it.