Add reqtest, reqhtml; deprecate requests.ToHTML and testing transports (

#115)

* Add reqtest, reqhtml; deprecate requests.ToHTML and testing transports

* reqtest: Add package doc

* reqtest: Rename to reqtest.Server and improve docs

* reqhtml.Body: Better doc string

* Docs: Better docs for reqtest.Replay.

Docs: Deprecate ToHeaders

More cleanup of ExampleBuilder_BodyJSON

ExampleError: More consistent struct

ExampleBuilder_Headers: More realistic example

Core: Move RequestBuilder into req.go

Rename TestServer → TestServerConfig

CI: Upgrade codeql

builder_core.go: Use generics to simplify

Go.mod: Upgrade x/net