tests: Use unique name for TEST_PATH
We had a cases in patch where two tests were using the same TEST_PATH
and therefore they were stepping each other to the same files
which caused failures. These failures are not easy to reproduce.
This patch uses macro BASE_FILE_STEM for unique name. It should prevent
copy&paste problem resulting to intermittent failures.
Reviewed-by: Jakub Hrozek <email@example.com>