Add the ability to set in-testsuite. As with bug resolution and milestone, this must operate on all instances of a bug, not particular bug/changeset instances.
I think this would be manual - m-cMerge shouldn't make any guesses. We can get a list of affected filenames from hg.m.o, but that alone isn't sufficient. If a Makefile and a file of the test_* form are both touched, then yes, it could well be a test has been added. Equally though, it may be the disabling of a test for randomorange.
We would thus need to keep the list of modified files around, show the modified files in the UI, add a checkbox, and add the relevant field to the bug data and submission.