[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

Author: kastiglione

lldb/test/API/functionalities/exec/TestExec.py

@@ -102,13 +102,18 @@ def cleanup():
             # Run and we should stop at breakpoint in main after exec
             process.Continue()
 
+        self.assertState(process.GetState(), lldb.eStateStopped)
+        for t in process.threads:
+            if t.stop_reason != lldb.eStopReasonNone:
+                self.assertStopReason(t.stop_reason, lldb.eStopReasonBreakpoint,
+                    "Unexpected stop reason")
+                if self.TraceOn():
+                    print(t)
+                    if t.stop_reason != lldb.eStopReasonBreakpoint:
+                        self.runCmd("bt")
+
         threads = lldbutil.get_threads_stopped_at_breakpoint(
             process, breakpoint2)
-        if self.TraceOn():
-            for t in process.threads:
-                print(t)
-                if t.GetStopReason() != lldb.eStopReasonBreakpoint:
-                    self.runCmd("bt")
         self.assertEqual(len(threads), 1,
                         "Stopped at breakpoint in exec'ed process.")