Port Client connect/pool tests to hyper-util

[seanmonstar]

The integration tests checking the Connector and Pool parts of Client should be ported to the hyper-util crate. Not necessarily copy and paste, but that each logical case is tested. They were here in 0.14.x.

[Michael-J-Ward]

Hi Sean,

My understanding from the docs is that this Pool will serve a replacement for the old Client, per the roadmap.

hyper/docs/ROADMAP.md

Lines 296 to 307 in 984760f

	### client
	A `Pool` struct that implements `Service` is provided. It fills a similar role
	as the previous `hyper::Client`.
	> **Note**: The `Pool` might be something that goes into the `tower` crate
	> instead. Or it might stay here as a slightly more specialized racing-connect
	> pool. We'll find out as we go.
	A `connect` submodule that mostly mirrors the existing `hyper::client::connect`
	module is moved here. Connectors can be used as a source to provide `Service`s
	used by the `Pool`.

Following in the tower discord channel, it looks like you may be experimenting with the impl Service<Req> for Pool<T>, and I think that implementation blocks this test-porting. Is that correct?

PS: Sorry if I goaded you into closing this issue too early.