t7408: replace incorrect uses of test_must_fail

According to t/README, test_must_fail() should only be used to test for
failure in Git commands.

Replace the invocation of `test_must_fail test_path_is_file` with
`test_path_is_missing` since, in this test case, the path should not
exist at all.

In all the cases where `test_must_fail test_alternate_is_used` appears,
test_alternate_is_used() fails because test_line_count() cannot open the
non-existent $alternates_file. Replace
`test_must_fail test_alternate_is_used` with `test_path_is_missing` to
test for this directly.

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

t/t7408-submodule-reference.sh

@@ -122,8 +122,8 @@ test_expect_success 'missing submodule alternate fails clone and submodule updat
 		# update of the submodule succeeds
 		test_must_fail git submodule update --init &&
 		# and we have no alternates:
-		test_must_fail test_alternate_is_used .git/modules/sub/objects/info/alternates sub &&
-		test_must_fail test_path_is_file sub/file1
+		test_path_is_missing .git/modules/sub/objects/info/alternates &&
+		test_path_is_missing sub/file1
 	)
 '
 
@@ -137,7 +137,7 @@ test_expect_success 'ignoring missing submodule alternates passes clone and subm
 		# update of the submodule succeeds
 		git submodule update --init &&
 		# and we have no alternates:
-		test_must_fail test_alternate_is_used .git/modules/sub/objects/info/alternates sub &&
+		test_path_is_missing .git/modules/sub/objects/info/alternates &&
 		test_path_is_file sub/file1
 	)
 '
@@ -182,7 +182,7 @@ check_that_two_of_three_alternates_are_used() {
 	# immediate submodule has alternate:
 	test_alternate_is_used .git/modules/subwithsub/objects/info/alternates subwithsub &&
 	# but nested submodule has no alternate:
-	test_must_fail test_alternate_is_used .git/modules/subwithsub/modules/sub/objects/info/alternates subwithsub/sub
+	test_path_is_missing .git/modules/subwithsub/modules/sub/objects/info/alternates
 }