Minor fixes to readable-streams tests for ShadowRealm

[ptomato]

A few fixes for readable-streams tests with incorrect expectations for ShadowRealms:

• Don't execute the test that depends on VideoFrame in ShadowRealm, since VideoFrame isn't exposed there.
• For the test resource file that depends on setInterval, provide a "fake" implementation if setInterval/clearInterval aren't present on the global object, which is the case in ShadowRealm.

Reported by @mgaudet in https://github.com/tc39/proposal-shadowrealm/issues/396

[ptomato]

Thanks for the review! Should have all of the feedback incorporated now.

[ptomato]

Appreciate the reviews!