t2107: modernize path existence check

Replace '! test -f' with 'test_path_is_missing' for better
debugging information when the assertion fails.

Found using: git grep "test -[efd]" t/t????-*.sh

Signed-off-by: aditya <adityabnw07@gmail.com>

Author: a25781623-lang

t/t2107-update-index-basic.sh

@@ -86,7 +86,7 @@ test_expect_success '.lock files cleaned up' '
 	# the_index.cache_changed is zero, rollback_lock_file fails
 	git update-index --refresh --verbose >out &&
 	test_must_be_empty out &&
-	! test -f .git/index.lock
+	test_path_is_missing .git/index.lock
 	)
 '