t7110: replace `test -f` with `test_path_is_*` helpers

t7110: replace `test -f` with `test_path_is_*` helpers

`test -f` and `! test -f` do not provide clear error messages when they fail.
To enhance debuggability, use `test_path_is_file` and `test_path_is_missing`,
which instead provide more informative error messages.

Note that `! test -f` checks if a path is not a file, while
`test_path_is_missing` verifies that a path does not exist. In this specific
case the tests are meant to check the absence of the path, making
`test_path_is_missing` a valid replacement.

Signed-off-by: Matteo Bagnolini <matteobagnolini2003@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>