Test: cts-scheduler: Improve file name handling

Use variables for subdirectory names, and use os.path.join() to
concatenate file paths in run_one().

In the future, the following would be nice:

• Use more consistent variable names for files/dirs.
• Group input and output dirs into a dict or class, instead of using a plain variable for each.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>