providers:checkbox: Set PYTHONPATH when invoking piglit tests (LP: #1400769)

This is a kludgy workaround for the linked bug, while a proper solution
is implemented by piglit upstream.
providers:checkbox: PEP8 fixes
"automatic merge of lp:~roadmr/checkbox/1400769-piglit-pythonpath/ by tarmac [r=sylvain-pineau][bug=1400769][author=roadmr]"