t7101: modernize test path checks

t7101: modernize test path checks

Replace old-style `test -[df]` and `! test -[df]` assertions with
the modern `test_path_is_file`, `test_path_is_dir`, and
`test_path_is_missing` helpers.

These helpers provide more informative error messages in case of
failure (e.g., "File 'foo' is missing" instead of just exit code 1).

While at it, fix a typo and an incorrect path
reference in one of the test descriptions.

Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>