transform WPT <to-javascript-url-script-src.html>'s async sub-tests to promise-tests and remove checking the securitypolicyevent's target element.
Gecko doesn't set the securitypolicyevent violation's target element,
which prevented the async tests from passing with Gecko.
Setting the violation's target element isn't specified (
w3c/webappsec-csp#687).
Promise-based tests allow removing checking the target element, because
the tests are run in sequence
(https://web-platform-tests.org/writing-tests/testharness-api.html#promise-tests).
Differential Revision: https://phabricator.services.mozilla.com/D227160
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1919248
gecko-commit: b941985be92395c909b233ecfbba356f6d0db149
gecko-reviewers: smaug