t3200: clean up checks for file existence

t3200: clean up checks for file existence

This patch uses test_path_is_file and test_path_is_missing
instead of "test -f / ! test -f" checks. The former are more
verbose in case of failure and more precise (e.g., is_missing
will check that the entry is actually missing, not just not
a regular file).

As a bonus, this also fixes a few buggy tests that used
"test foo" instead of "test -f foo", and consequently always
reported success.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>