Workaround pypy bug on trusty when running tests

https://bitbucket.org/pypy/pypy/issue/1694/ affects zaqar when running
pypy tests on trusty slave nodes. We can work around that bug by setting
--jit off when running the pypy interpreter. Do this until the upstream
bug fix is backported into trusty.

Change-Id: Iebaa167e9e12bbc1017371929ca8c182bacc2aad
Related-Bug: 1368418
Merge "Workaround pypy bug on trusty when running tests"