nts: allow missing SIV support

When compiled with NTS support, don't require a SIV cipher to be always
supported (e.g. due to a different version of a library used for
building). Handle this case with a fatal message instead of crash.
Also, check the support early in the client unit test to prevent a hang.

test/unit/nts_ntp_client.c

@@ -200,6 +200,8 @@ test_unit(void)
   IPAddr ip_addr;
   int i, j, prev_num_cookies, valid;
 
+  TEST_CHECK(SIV_GetKeyLength(AEAD_AES_SIV_CMAC_256) > 0);
+
   SCK_GetLoopbackIPAddress(AF_INET, &addr.ip_addr);
   addr.port = 0;