[Dexter] Add os.path.normcase(...) transform to test path early.
When passing a test path, if the path points directly at a file, then normcase would not be called on path. This would change the expected lower case drive path, on windows, to be uppercase. This patch simply calls normcase on the test path at the earliest point possible to avoid this issue. Reviewers: djtodoro, jmorse Differential Revision: https://reviews.llvm.org/D78633