Change WPT test for File constructor replacement.

Change the test to follow the proposed change in w3c/FileAPI#41,
now that all browsers agree to not replace /.

Depends on D86981

Differential Revision: https://phabricator.services.mozilla.com/D87112

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1650607
gecko-commit: 365ae38a5bca68e4d8c8cf25b8f5230e306b0f09
gecko-integration-branch: autoland
gecko-reviewers: baku

FileAPI/file/File-constructor.html

@@ -84,7 +84,8 @@
 }
 
 test_second_argument("dummy", "dummy", "Using fileName");
-test_second_argument("dummy/foo", "dummy:foo", "Using special character in fileName");
+test_second_argument("dummy/foo", "dummy/foo",
+                     "No replacement when using special character in fileName");
 test_second_argument(null, "null", "Using null fileName");
 test_second_argument(1, "1", "Using number fileName");
 test_second_argument('', '', "Using empty string fileName");