test: drop logging suspension

Instead of selectively suspending logging by redirecting messages to
/dev/null, increase the default minimum log severity to FATAL. In the
debug mode, all messages are printed.

test/unit/ntp_core.c

@@ -360,10 +360,7 @@ test_unit(void)
   NIO_Initialise();
   NCR_Initialise();
   REF_Initialise();
-
-  TST_SuspendLogging();
   KEY_Initialise();
-  TST_ResumeLogging();
 
   CNF_SetupAccessRestrictions();