Replace check_log_err with log_includes/log_excludes

The latter are more explicit and easier to understand at the call site.

Also removed the redundant crash checks via the log files.

maxscale-system-test/mxs657_restart_service.cpp

@@ -81,7 +81,6 @@ int main(int argc, char* argv[])
     sht_rst_service();
 
     Test->check_maxscale_alive(0);
-    Test->check_log_err(0, (char*) "received fatal signal", false);
     int rval = Test->global_result;
     delete Test;
     return rval;