assert: validate the block return type

This makes sure the returned value when calling block is actually of type promise in case assert.rejects or assert.doesNotReject is called.

This currently relies on #19885 and I am going to rebase as soon as that lands / changes. Marking as blocked in the meanwhile.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines