t0001: use test_path_is_*

t0001: use test_path_is_*

t0001 predates the test_path_is_* helpers, and uses "test
-f" and "test -d" directly. Using the helpers provides
better debugging output, and are a little more robust.
As opposed to "! test -d", test_path_is_missing will
actually makes sure the path does not exist at all.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>