[lit] Factor out listdir logic shared by different test formats.

Summary:
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.

Reviewers: zturner, MatzeB, modocache

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D34855

llvm-svn: 306895