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/bug664.cpp

@@ -12,6 +12,6 @@ int main(int argc, char* argv[])
 {
     TestConnections test(argc, argv);
     test.check_maxscale_alive(0);
-    test.check_log_err(0, "Failed to create new router session for service 'RW_Split'", true);
+    test.log_includes(0, "Failed to create new router session for service 'RW_Split'");
     return test.global_result;
 }