Validate API examples. Rename files

> If you define a rule for a file which doesn't exist the test won't fail. 
Actually, the `len(files) == len(expected)` would catch this. This https://github.com/marianitadn/origin/blob/test-examples/examples/examples_test.go#L128 already compares the file names.

If you wanted to print out the explicit "extra" file name, you could do that right in the walk-through function (https://github.com/marianitadn/origin/blob/test-examples/examples/examples_test.go#L101).