t1401: remove lockfile creation

To create error conditions, some tests set up reference locks by
directly creating its lockfile. While this works for the files reference
backend, this approach is incompatible with the reftable backend.
Refactor the test to create a d/f conflict via git-symbolic-ref(1)
instead so that the test is reference backend agnostic.

Signed-off-by: Justin Tobler <[email protected]>
Acked-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jltobler

t/t1401-symbolic-ref.sh

@@ -106,9 +106,8 @@ test_expect_success LONG_REF 'we can parse long symbolic ref' '
 '
 
 test_expect_success 'symbolic-ref reports failure in exit code' '
-	test_when_finished "rm -f .git/HEAD.lock" &&
-	>.git/HEAD.lock &&
-	test_must_fail git symbolic-ref HEAD refs/heads/whatever
+	# Create d/f conflict to simulate failure.
+	test_must_fail git symbolic-ref refs/heads refs/heads/foo
 '
 
 test_expect_success 'symbolic-ref writes reflog entry' '