test: do not negate test_path_is_* to assert absense
These tests use "! test_path_is_dir" or "! test_path_is_file" to
ensure that the path is not recursively checked out or "submodule
update" did not touch the working tree.
Use "test_path_is_missing" to assert that the path does not exist,
instead of negating test_path_is_* helpers; they are designed to be
loud in wrong occasions. Besides, negating "test_path_is_dir" would
mean we would be happy if a file exists there, which is not the case
for these tests.
Signed-off-by: Junio C Hamano <gitster@pobox.com>