[flang][driver] Merge test/Driver and test/Flang-Driver directories

Until now we've been maintaining 2 test directories for Flang's drivers:
  * test/Driver for `f18` (the current driver)
  * test/Flang-Driver for `flang-new` (the new driver)
As we have started sharing tests between the drivers, this separation is
no longer required. This patch merges the two test directories. As
suggested in the review, moving forward we'll avoid having tests
specifically for the old driver.

A few notable changes:
  * Driver/version-test.f90 and Driver/no-files.f90 are deleted. The
  versions for the new driver are more robust, but tricky to share.
  * Driver/write-module.f90 is deleted in favour of
    Flang-Driver/write-module.f90 (see https://reviews.llvm.org/D97197
    for more context)

Differential Revision: https://reviews.llvm.org/D98257