auth: Make luci-go services trust signatures produced by the token server.

server/auth/internal/fetch.go

Index: server/auth/internal/fetch.go
diff --git a/server/auth/internal/fetch.go b/server/auth/internal/fetch.go
index 672e4ff8eac16be10213e4d2915f9afd9c5ccca0..cb3512c9effbde805d735e779680b7666c9c2866 100644
--- a/server/auth/internal/fetch.go
+++ b/server/auth/internal/fetch.go
@@ -71,6 +71,9 @@ func (r *Request) Do(c context.Context) error {
 		}
 	} else {
 		client = http.DefaultClient
+		if testTransport := c.Value(&testTransportKey); testTransport != nil {
+			client = &http.Client{Transport: testTransport.(http.RoundTripper)}
+		}
 	}
 
 	// Prepare a blob with the request body. Marshal it once, to avoid