CCID_Tests: Ensure the response to "SELET MF" is two bytes (SW) long

we used to accept arbitrary length responses, which made us fail to
catch the bug described in OS#4684

Change-Id: I35721ce55b58a2f70ea30a0d1bd16d2ac58dfcab
Related: OS#4684

ccid/CCID_Tests.ttcn

@@ -379,7 +379,7 @@ private function f_TC_select_mf() runs on Slot_CT
 	f_ccid_power_on();
 	f_ccid_set_par(ts_ProtoDataT0(ISO7816_FI_512_5MHz, ISO7816_DI_32));
 	for (i := 0; i < 1000; i := i+1) {
-		f_ccid_xfr(c_UICC_SELECT_MF, ?);
+		f_ccid_xfr(c_UICC_SELECT_MF, '??'O);
 	}
 }
 testcase TC_select_mf() runs on Test_CT