Convert http testing functions to builder pattern

[twifkak]

There are a bunch of cryptically named functions to issue fake HTTP requests to a handler and record their responses:

amppackager/packager/testing/testing.go

Lines 92 to 122 in bf061be

	func Get(t *testing.T, handler http.Handler, target string) *http.Response {
	return GetH(t, handler, target, http.Header{})
	}
	func GetH(t *testing.T, handler http.Handler, target string, headers http.Header) *http.Response {
	return GetBHH(t, handler, target, "", nil, headers)
	}
	func GetHH(t *testing.T, handler http.Handler, target string, host string, headers http.Header) *http.Response {
	return GetBHH(t, handler, target, host, nil, headers)
	}
	func GetBHH(t *testing.T, handler http.Handler, target string, host string, body io.Reader, headers http.Header) *http.Response {
	rec := httptest.NewRecorder()
	method := ""
	if body != nil {
	method = "POST"
	headers.Set("Content-Type", "application/x-www-form-urlencoded")
	}
	req := httptest.NewRequest(method, target, body)
	for name, values := range headers {
	for _, value := range values {
	req.Header.Add(name, value)
	}
	}
	if host != "" {
	req.Host = host
	}
	handler.ServeHTTP(rec, req)
	return rec.Result()
	}

Instead, they should follow a builder pattern, something like Request(t, handler, "/blah").Headers({...}).Body("foo").Get(). That way the names are not unreadable, and we don't have a combinatorial explosion of the different variants of Get().