In a lot of our oe-selftest tests, for example in the devtool module, we use something like self.assertTrue(os.path.exists(filepath)) - this leads to slightly awkward messages on failure starting with "False is not true" (no kidding!). To avoid that and make the test code slightly less verbose, let's add assertExists() and assertNotExists() methods on the base test class and change all tests doing an assert on file existence to use that.