feat(client): allow tests to assign a fake requester

[flotter]

The client test suite used by all client command tests fakes the doer today, which is a private internal Requester implementation detail. Derived projects using the client Requester for extended commands needs to fake the Requester interface.

This PR adds a client method SetRequester(rw Requester) to allow test code to fake the requester. This has to be public to allow an external package to fake this.

Pending work remain to properly refactor the existing client test code to Fake the Requester instead of the doer. This is captured as an outstanding issue here: #315

[niemeyer]

So we spent all those calls to agree that we'd have no external requesters for now, and the very first trivial PR allows setting an external Requester, with no mention of these conversations? :)

[flotter]

I will take a slightly different approach as used elsewhere in Pebble:

func (s *BasePebbleSuite) RedirectClientToTestServer(handler func(http.ResponseWriter, *http.Request)) {
        server := httptest.NewServer(http.HandlerFunc(handler))
        s.BaseTest.AddCleanup(func() { server.Close() })
        cli.ClientConfig.BaseURL = server.URL
        s.BaseTest.AddCleanup(func() { cli.ClientConfig.BaseURL = "" })
}