[unittests] Add a helper function for getting an input file

Summary:
Fetching an input file required about five lines of code, and this was
repeated in multiple unit tests, with slight variations. Add a helper
function for doing that into the lldbUtilityMocks module (which I rename
to lldbUtilityHelpers to commemorate the fact it includes more than
mocks)

Reviewers: zturner, eugene

Subscribers: emaste, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D34683

llvm-svn: 306668