testing: add a pair of E303 testcases

Add two test cases to verify that pycodestyle correctly flags 2 empty
lines in non-toplevel contexts, even outside of functions: one for
two empty lines between class methods and one for two empty lines in the
intended body of an `if:` statement at toplevel.

testing/data/E30.py

@@ -75,6 +75,20 @@ def a():
 #:
 
 
+#: E303:6:5
+class xyz:
+    def a(self):
+        pass
+
+
+    def b(self):
+        pass
+#: E303:5:5
+if True:
+    a = 1
+
+
+    a = 2
 #: E304:3:1
 @decorator