t9160: use test_path_is_missing()

The test_must_fail() function should only be used for git commands since
we assume that external commands work sanely. Since, not only should
this file not exist, but there shouldn't exit _any_ filesystem entity in
these paths, replace `test_must_fail test -f` with
`test_path_is_missing`.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9160-git-svn-preserve-empty-dirs.sh

@@ -86,8 +86,8 @@ test_expect_success 'remove non-last entry from directory' '
 		cd "$GIT_REPO" &&
 		git checkout HEAD~2
 	) &&
-	test_must_fail test -f "$GIT_REPO"/2/.gitignore &&
-	test_must_fail test -f "$GIT_REPO"/3/.gitignore
+	test_path_is_missing "$GIT_REPO"/2/.gitignore &&
+	test_path_is_missing "$GIT_REPO"/3/.gitignore
 '
 
 # After re-cloning the repository with --placeholder-file specified, there