Fix a bug in PythonExceptionState and add unittest coverage.

I forgot to reset the restore flag when calling member function
`Acquire`.  The newly added unittest should cover this case.

llvm-svn: 253002

lldb/source/Plugins/ScriptInterpreter/Python/PythonExceptionState.h

@@ -30,6 +30,9 @@ class PythonExceptionState
     void
     Discard();
 
+    void
+    Reset();
+
     static bool
     HasErrorOccurred();