test: remove support for suffix in get_testdata_dir()
Instead, use path_join() in callers wherever needed.
(cherry picked from commit 55890a40c3ec0c061c04d1395a38c26313132d12)
Related: #1763435
patch_name: 0405-test-remove-support-for-suffix-in-get_testdata_dir.patch
present_in_specfile: true
location_in_specfile: 405
squash_commits: true