[lldb] avoid assert in threadsanitizer tests on linux

The tests are unsupported on linux, but they assert in
Thread::GetStopDescriptionRaw() because of empty stop reason
description. And it is empty because
InstrumentationRuntimeTSan::NotifyBreakpointHit() fails
to get report from InstrumentationRuntimeTSan::RetrieveReportData(),
which is possibly(?) the reason why this is unsupported on linux.
Add a dummy stop reason description for this case, which changes
the test result from failing to unsupported.