The lit test formats use largely the same logic for discovering tests. There are
some superficial differences in the logic, which seem reasonable enough to
handle in a single routine.
At a high level, the common goal is "look for files that end with one of these
suffixes, and skip anything starting with a dot." The balance of the logic
specific to ShTest and GoogleTest collapses quite a bit, so that
getTestsInDirectory is only a couple of lines around a call to the new function.