test: add failing http `agent: null` test

See #7012.
http, https: don't depend on `globalAgent`

For the `request()` and `get()` functions. I could never
really understand why these two functions go through agent
first... Especially since the user could be passing `agent: false`
or a different Agent instance completely, in which `globalAgent`
will be completely bypassed.

Moved the relevant logic from `Agent#request()` into the
`ClientRequest` constructor.

Incidentally, this commit fixes #7012 (which was the original
intent of this commit).
test: update "http-*" tests to only use public API

Don't invoke the `agent.requst()` or `agent.get()` functions
directly. Instead, use the public API and pass the agent
instance in as the `agent` option.
http: remove the circular dependency

Between `ClientRequest` and `Agent`. The circular require was doing
weird things at load time, like making the `globalAgent` property
be `undefined` from within the context of the "_http_client"
module.

Removing the circular dependency completely fixes this.

This commit effectively removes the undocumented `Agent#request()`
and `Agent#get()` functions.