[lldb] Add more asserts to TestExec

If this test fails, the error message isn't helpful:

```
self.assertEqual(len(threads), 1,
AssertionError: 0 != 1 : Stopped at breakpoint in exec'ed process
```

This change adds asserts to verify that:
1. The process is stopped
2. For each thread, that the stop reason is None, or Breakpoint

The latter will indicate if execution has stopped for some other reason, and if so what
that reason is.

Differential Revision: https://reviews.llvm.org/D148588