[test] update unit test emulating radio (#9200)

This commit updates the unit tests that emulate the radio platform API
to define `otPlatRadioGetCaps()`, which returns the ACK_TIMEOUT and
CSMA capabilities. This is to avoid having CSMA being implemented by
the `SubMac`, which then uses the microsecond timer that is not
emulated in the unit test platform.

It also updates the `InitTest()` function to clear the radio platform
variable `sRadioTxOngoing`. This is to avoid the possible
(though unlikely) situation where an earlier test finishes in the
middle of a transmission, causing an extra `TxDone` event in the
following test.

tests/unit/test_srp_server.cpp

@@ -80,6 +80,8 @@ void AdvanceTime(uint32_t aDuration);
 
 extern "C" {
 
+otRadioCaps otPlatRadioGetCaps(otInstance *) { return OT_RADIO_CAPS_ACK_TIMEOUT | OT_RADIO_CAPS_CSMA_BACKOFF; }
+
 otError otPlatRadioTransmit(otInstance *, otRadioFrame *)
 {
     sRadioTxOngoing = true;
@@ -188,10 +190,12 @@ void InitTest(void)
     // Initialize OT instance.
 
     sNow      = 0;
+    sAlarmOn  = false;
     sInstance = static_cast<Instance *>(testInitInstance());
 
     memset(&sRadioTxFrame, 0, sizeof(sRadioTxFrame));
     sRadioTxFrame.mPsdu = sRadioTxFramePsdu;
+    sRadioTxOngoing     = false;
 
     //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     // Initialize Border Router and start Thread operation.