Ignore state directory in suid plugins

If a plugin still has suid privileges at the time np_enable_state() is
called, the MP_STATE_DIRECTORY environment will be ignored.

There is no need for a NEWS entry as no suid plugins use np_enable_state
yet. - Thanks dermoth (https://github.com/monitoring-plugins/monitoring-plugins/commit/68fe713335183dd37ced78821711c2a3f1ea2cc7)
Add tests for negative thresholds in test_utils

Thanks Dermoth - (https://github.com/monitoring-plugins/monitoring-plugins/commit/bc92b9392009766441c67b06398e6925cc45c2aa)