t7301: use test_path_is_(missing|file)

Replace "test -f" and friends to use the test_path_is_file helper
function and friends from test-lib-functions.sh. These functions
perform identical operations while enhancing debugging capabilities
in case of test failures.

The original used 'test ! -f' to check if the file has been
correctly cleaned, so 'test ! -e' would have been a better choice.
Replace them with 'test_path_is_missing'.

Signed-off-by: Vincenzo Mezzela <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: vmezzela