Add tests for the filename.

Test that the stacklevel is handled correctly.
diff --git a/Lib/test/warning_tests.py b/Lib/test/warning_tests.py
new file mode 100644
index 0000000..d0519ef
--- /dev/null
+++ b/Lib/test/warning_tests.py
@@ -0,0 +1,9 @@
+# Helper module for testing the skipmodules argument of warnings.warn()
+
+import warnings
+
+def outer(message, stacklevel=1):
+    inner(message, stacklevel)
+
+def inner(message, stacklevel=1):
+    warnings.warn(message, stacklevel=stacklevel)