tests: Add testcases for catching user Exception subclasses.

tests/basics/subclass-native3.py

@@ -5,3 +5,18 @@ e = MyExc(100, "Some error")
 print(e)
 print(repr(e))
 print(e.args)
+
+try:
+    raise MyExc("Some error")
+except MyExc as e:
+    print("Caught exception:", repr(e))
+
+try:
+    raise MyExc("Some error2")
+except Exception as e:
+    print("Caught exception:", repr(e))
+
+try:
+    raise MyExc("Some error2")
+except:
+    print("Caught user exception")