t2021: do not negate test_path_is_dir

In this test, a path (some_dir) that is originally a directory is to be removed and then to be replaced with a file of the same name. The expectation is that the path becomes a file at the end. However, "! test_path_is_dir some_dir" is used to catch a breakage that leaves the path as a directory. But as with all the "test_path_is..." helpers, this use of the helper makes it loud when the expectation (i.e. it is a directory) is met, and otherwise is silent when it is not---this does not help debugging. Be more explicit and state that we expect the path to become a file. Signed-off-by: Junio C Hamano <gitster@pobox.com>