This patch takes the idea a little further, adding a command line option to regrtest, '--failnoimpl' (it could use a better name).

With this option specified any test decorated by support.not_implemented will not be marked as an expected failure, allowing the NotImplementedError exception to fail the test.  The default under regrtest is to expect the failure, but the default for running a test file directly is to fail loudly.  The idea is just to make it more obvious when a test is just in need of implementation, and could make it easier for new contributors to find something that needs doing.