http2: cover server recv SETTINGS

http2_test.go

@@ -115,6 +115,12 @@ func TestServer(t *testing.T) {
 	})
 	defer st.Close()
 
+	covers("3.5", `
+		The server connection preface consists of a potentially empty
+		SETTINGS frame ([SETTINGS]) that MUST be the first frame the
+		server sends in the HTTP/2 connection.
+	`)
+
 	st.writePreface()
 	st.writeInitialSettings()
 	st.wantSettings().ForeachSetting(func(s Setting) {