Convert subunit logs to subunit v2 before use.

* modules/jenkins/files/slave_scripts/run-tox.sh: Feeding subunit v1
through a subunit v2 parser then running the leftovers through a subunit
v1 parser is a really slow way to support subunit v1 and subunit v2.
Instead convert subunit v1 to subunit v2 to speed up parsing. Local
testing indicates this should shave off minutes of test run times.

Closes-Bug: #1221094
Change-Id: Ie17708ac217e7468bb1c92cfda7aa50994178817