t3507: use test_path_is_missing()

The test_must_fail() function should only be used for git commands since
we should assume that external commands work sanely. Replace
`test_must_fail test_path_exists` with `test_path_is_missing` since we
expect these paths to not exist.

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

t/t3507-cherry-pick-conflict.sh

@@ -168,7 +168,7 @@ test_expect_success 'successful final commit clears cherry-pick state' '
 	echo resolved >foo &&
 	test_path_is_file .git/sequencer/todo &&
 	git commit -a &&
-	test_must_fail test_path_exists .git/sequencer
+	test_path_is_missing .git/sequencer
 '
 
 test_expect_success 'reset after final pick clears cherry-pick state' '
@@ -178,7 +178,7 @@ test_expect_success 'reset after final pick clears cherry-pick state' '
 	echo resolved >foo &&
 	test_path_is_file .git/sequencer/todo &&
 	git reset &&
-	test_must_fail test_path_exists .git/sequencer
+	test_path_is_missing .git/sequencer
 '
 
 test_expect_success 'failed cherry-pick produces dirty index' '
@@ -387,7 +387,7 @@ test_expect_success 'successful final commit clears revert state' '
 	echo resolved >foo &&
 	test_path_is_file .git/sequencer/todo &&
 	git commit -a &&
-	test_must_fail test_path_exists .git/sequencer
+	test_path_is_missing .git/sequencer
 '
 
 test_expect_success 'reset after final pick clears revert state' '
@@ -397,7 +397,7 @@ test_expect_success 'reset after final pick clears revert state' '
 	echo resolved >foo &&
 	test_path_is_file .git/sequencer/todo &&
 	git reset &&
-	test_must_fail test_path_exists .git/sequencer
+	test_path_is_missing .git/sequencer
 '
 
 test_expect_success 'revert conflict, diff3 -m style' '