increase test coverage for async-await

[nikomatsakis]

As part of the async-await feature we want to make sure we have good test coverage. We have a master list of tests prepared by @cramertj and @Centril that we'd like to complete.

[cramertj]

#62106 is a first step towards this.

[Centril]

• generalize impl trait to permit multiple lifetime bounds #61775 (comment)
• Testing coverage for control flow operators especially ?.
• Tests for async unsafe fn.