Increase slow_unit_tests timeout from 300s to 600s

Sets the test timeout for slow_unit_tests jobs to 600 seconds, replacing the default of 300 seconds.

This can potentially be removed if we either increase the nodes in our CI clusters or pursue #221 (closed) and test in isolated CI containers against local etcd and kube-apiserver.

Closes #246 (closed)

Closes #247 (closed)