control/controlclient: restore Options.HTTPC as Options.HTTPTestClient

I removed the HTTPC field in b6fa5a69be but it was apparently still
used in [oss-skipped] tests.

Restore it, but name it so it's more obvious that it's only for
tests. (It currently is, and I'd like to keep it like that for now.)

control/controlclient/direct.go

@@ -102,6 +102,7 @@ type Options struct {
 	NewDecompressor func() (Decompressor, error)
 	KeepAlive       bool
 	Logf            logger.Logf
+	HTTPTestClient  *http.Client // optional HTTP client to use (for tests only)
 }
 
 type Decompressor interface {
@@ -128,10 +129,13 @@ func NewDirect(opts Options) (*Direct, error) {
 		opts.Logf = log.Printf
 	}
 
+	httpc := opts.HTTPTestClient
+	if httpc == nil {
 		tr := http.DefaultTransport.(*http.Transport).Clone()
 		tr.ForceAttemptHTTP2 = true
 		tr.TLSClientConfig = tlsdial.Config(serverURL.Host, tr.TLSClientConfig)
-	httpc := &http.Client{Transport: tr}
+		httpc = &http.Client{Transport: tr}
+	}
 
 	c := &Direct{
 		httpc:           httpc,