Previously, rustdoc would issue a delay_span_bug ICE on the following code:
```rust
pub fn a() -> impl Fn() -> u32 {
|| content::doesnt::matter()
}
```
This wasn't picked up earlier because having `type Alias = impl Trait;`
in the same module caused _all closures_ to be typechecked, even if they
wouldn't normally. Additionally, if _any_ error was emitted, no
delay_span_bug would be emitted. So as part of this commit all of the
tests were separated out into different files.
