Stop comparing tests counts

* modules/jenkins/files/slave_scripts/run-tox.sh: This didn't work
reliably, and also becomes very hard to get right between system and
non-system python interpreters. Also undo workarounds we added
earlier in the 983ca11 change.

Change-Id: I48ddfb908b6a7cae9751ecc0c8b7fc56ed5a5e50