[lldb][test][FreeBSD] Fix some concurrent event tests (#84155)

A lot of `TestConcurrent*.py` expect one of the threads to crash, but we
weren't checking for it properly.

Possibly because signal reporting got better on FreeBSD at some point,
and it now shows the same info as Linux does.

```
  lldb-api :: functionalities/inferior-changed/TestInferiorChanged.py
  lldb-api :: functionalities/inferior-crashing/TestInferiorCrashing.py
  lldb-api :: functionalities/inferior-crashing/TestInferiorCrashingStep.py
  lldb-api :: functionalities/inferior-crashing/recursive-inferior/TestRecursiveInferior.py
  lldb-api :: functionalities/inferior-crashing/recursive-inferior/TestRecursiveInferiorStep.py
  lldb-api :: functionalities/thread/concurrent_events/TestConcurrentCrashWithBreak.py
  lldb-api :: functionalities/thread/concurrent_events/TestConcurrentCrashWithSignal.py
  lldb-api :: functionalities/thread/concurrent_events/TestConcurrentCrashWithWatchpoint.py
  lldb-api :: functionalities/thread/concurrent_events/TestConcurrentCrashWithWatchpointBreakpointSignal.py
```

Fixes #48777

`TestConcurrentTwoBreakpointsOneSignal.py` no longer fails, at least on
an AWS instance, so I've removed the xfail there.