Currently, the .arcunit files use this rule:
However, this means that the tests don't run if you only touch unit test data files which don't end in .php. For example, in D13992 I touched two test cases that are in .lint-test files, but that touch didn't trigger the tests to run on arc unit. It seems like it should, since I directly changed the tests and the standard rules can discover the correct tests to run on this data.
We could use the rule (^src/) instead, or possibly just (.*) (or maybe we can omit it entirely, I don't recall the behavior of no rule).
See some prior discussion in D13853. Maybe (.*) is best?