Correct wrongly named environment property
Fix #928: Increase partest thread pool timeout to 5 hours

Currently, a successful full nightly build takes between 3:30 and 4:00
hrs to run (depending on other load on the CI system). The default
partest thread pool timeout (4h) is therefore too tight and probably
made some builds fail unnecessarily. We increase the timeout to 5h to
allow for some slack.
Merge pull request #943 from gzm0/improve-partest

Improve partest