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/mxs1543.cpp

@@ -26,7 +26,7 @@ int main(int argc, char** argv)
 
     // Wait for the avrorouter to process the data
     sleep(10);
-    test.check_log_err(0, "Possible STATEMENT or MIXED", true);
+    test.log_includes(0, "Possible STATEMENT or MIXED");
 
     test.revert_replicate_from_master();