MT mode not reset to false in some tests

Found on FreeBSD 11.4 but not bound to the system, just to the order dhcpsrv library unit tests are run.

Got when running enough unit tests (so all, P* but not ParseConfigTest*) a failure on ParseConfigTest hook unit tests. Logs show these were from not multi-threading compatible hooks. My conclusion is the multi-threading mode was left to true by a preceding unit test (Pg* one for P*) and when I added an explicit set to false for the multi-threading mode in ParseConfigTest fixture constructor the bug was fixed.

So I propose as an emergency measure to add this fix and in a long term measure to create a RAII setter for the multi-threading mode so no it can't be left to true by accident. This can be added to the multi-threading or a new testutils library header.