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/bug547.cpp

@@ -53,7 +53,6 @@ int main(int argc, char* argv[])
     test.repl->unblock_all_nodes();
 
     test.stop_timeout();
-    test.check_log_err(0, "fatal signal 11", false);
 
     return test.global_result;
 }