Add a Trusted Type test for setting ShadowRoot's innerHTML.
This is essentially a copy of
block-string-assignment-to-ShadowRoot-setHTMLUnsafe.html, replacing
set shadowRoot.setHTMLUnsafe(html) with shadowRoot.innerHTML = html.
Note that innerHTML uses LegacyNullToEmptyString, so setting it to
a null behaves slightly differently.
Differential Revision: https://phabricator.services.mozilla.com/D229149
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1931276
gecko-commit: 4b6f5fdc1a0ddf5ebd66be2d36743ac965489599
gecko-reviewers: smaug