[PATCH v2 2/2] t9803: update commit messages and description

From: Sanchit Jindal <sanchit1053@gmail.com>

Using the `test` function directly does not provide any diagnostic information to the user, in case of a failure.

This patch replace the `test` function with the helper functions defined that will call the corresponding test function and log an error message if the assert fails

Making the replacements
- `test -e` -> test_path_exists,
- `test ! -e` -> test_path_is_missing
- `test -f` -> test_path_is_file

Signed-off-by: Sanchit Jindal <sanchit1053@gmail.com>