[flang][nfc] Relocate a few driver tests

When the work on the Flang driver started, we created 2 test
directories:
  * flang/test/Frontend/
  * flang/test/Driver/
That was mostly done to model what Clang was doing. In practice, we
stopped using "flang/test/Frontend/" early on and most Flang driver
tests are currently located in "flang/test/Driver/". This patch moves
the remaining tests from the latter into the former directory.

This change also means that we can re-use test input files, i.e.
flang/test/Frontend/Inputs/hello-world.f90 can be replaced with
flang/test/Driver/Inputs/hello.f90. To this end, the affected test is
updated (multiple-input-files.f90).

Differential Revision: https://reviews.llvm.org/D130633