tests: make use of helpers to create unique filenames

Start to use the helpers introduced in "tests: add helpers to create
unique filenames".

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

Pekka: Dropped program name from output_filename_for_test_case() calls
as it is already added automatically.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>