t9803: Update commit messages and description

replacing `test -e` with test_path_exists,
	  `test ! -e` with test_path_is_missing
	  `test -f` with test_path_is_file
These helper functions will run the `test` command with the
corresponding flags and will echo a message if the assert fails.
This will provide better debugging logs for test, instead of the
previous method which provided no message

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