I am adding 2.7.  It is ok to add tests to the stable series release AFAIK.  Moreover, I believe the unittests should be written for 2.7 first.  Since 3.x port of trace was done without the benefit of a test suite, it is likely that there are many differences in behavior that should be eliminated.  (See issue9317.)

Please start with fine-graned "white-box" API tests.  These are easier because you don't need to set up external scripts, capture output, etc. Just create a Trace object in setUp and call its various methods in test cases.