Bug 1294075 - Disable marionette accessibility test on headless.

testing/marionette/harness/marionette_harness/tests/unit/unit-tests.ini

@@ -4,6 +4,7 @@
 [test_session.py]
 [test_capabilities.py]
 [test_accessibility.py]
+skip-if = headless # Bug 1294075 and 1341493
 [test_expectedfail.py]
 expected = fail
 [test_click.py]