Add snippet to check that int.__init__ doesn't mutate int.

tests/snippets/numbers.py

@@ -0,0 +1,10 @@
+x = 5
+x.__init__(6)
+assert x == 5
+
+class A(int):
+    pass
+
+x = A(7)
+assert x == 7
+assert type(x) is A