Sync file handling logic in RewriterTestContext.h and RefactoringTest.cpp.

They are mostly duplicated and got out of sync during the PathV1 removal. We
should factor the code somewhere, but for now a FIXME will do.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185019 91177308-0d34-0410-b5e6-96231b3b80d8