[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