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/bug626.cpp

@@ -27,7 +27,7 @@ int main(int argc, char* argv[])
     mysql_close(conn);
 
     execute_query(test.repl->nodes[0], "DROP USER 'old'@'%%'");
-    test.check_log_err(0, "MaxScale does not support these old passwords", true);
+    test.log_includes(0, "MaxScale does not support these old passwords");
 
     return test.global_result;
 }