Add test to <block-string-assignment-to-Element-setAttribute.html> which checks trusted types can be assigned to non-injection sinks

[mbrodesser-Igalia]

Mentioned at https://searchfox.org/mozilla-central/rev/c26f7461fc2a51196b7f517c7f98a1e271dc9ec0/testing/web-platform/tests/trusted-types/block-string-assignment-to-Element-setAttribute.html#92.

If so:

• why? It seems unnecessary.
• tests for that are needed.

[mbrodesser-Igalia]

E.g. https://jsfiddle.net/6hj2xra7/

[mbrodesser-Igalia]

https://w3c.github.io/trusted-types/dist/spec/#trusted-types mentions it's intended, so the test needs to be adapted.