Testsuite tweaks and refactoring

- Rename requires_th to req_th for consistency with other req functions
  (e.g. req_interp, req_profiling etc.)

- req_th (previously requires_th) now checks for interpreter (via
  req_interp). With this running TH tests are skipped when running the
  test suite with stage=1.

- Test tweaks:
    - T9360a, T9360b: Use req_interp
    - recomp009, T13938, RAE_T32a: Use req_th

- Fix check-makefiles linter: it now looks for Makefiles instead of .T
  files (which are actually Python files)