Add annotation to opt out of accessibility tests

Currently all WebDriver tests are checked for accessibility when run
through SFDC autobuilds. Several of our UI tests load test
components/apps that aren't accessible but we don't care since they're
just test apps. Using @CheckAccessibility(false) will prevent this
check.

@rev gtorres@
@rev mkohanfars@
@P4CL:9243212@
@bug W-2310377@
Merging origin/tbliss/master-pending-acessibility-ignore-flag into master