test: don't fail on other tests if test_po fails
test: don't fail on other tests if test_po fails

previously if this test failed, it would leave the current directory
in a inconstant state (pointing to a temporary directory that is
deleted).
This caused the following error on many tests in the testsuite:

cwd = os.getcwd()
OSError: [Errno 2] No such file or directory
yakky/django-cms,benzkji/django-cms,mkoistinen/django-cms,bittner/django-cms,czpython/django-cms,netzkolchose/django-cms,divio/django-cms,evildmp/django-cms,rsalmaso/django-cms,evildmp/django-cms,timgraham/django-cms,bittner/django-cms,yakky/django-cms,czpython/django-cms,benzkji/django-cms,divio/django-cms,jproffitt/django-cms,mkoistinen/django-cms,datakortet/django-cms,divio/django-cms,timgraham/django-cms,mkoistinen/django-cms,vxsx/django-cms,datakortet/django-cms,FinalAngel/django-cms,FinalAngel/django-cms,vxsx/django-cms,rsalmaso/django-cms,evildmp/django-cms,vxsx/django-cms,nimbis/django-cms,divio/django-cms,rsalmaso/django-cms,timgraham/django-cms,nimbis/django-cms,FinalAngel/django-cms,datakortet/django-cms,benzkji/django-cms,yakky/django-cms,evildmp/django-cms,jproffitt/django-cms,jproffitt/django-cms,bittner/django-cms,rsalmaso/django-cms,netzkolchose/django-cms,czpython/django-cms,netzkolchose/django-cms,bittner/django-cms,jsma/django-cms,jsma/django-cms,FinalAngel/django-cms,yakky/django-cms,czpython/django-cms,datakortet/django-cms,nimbis/django-cms,jsma/django-cms,jsma/django-cms,benzkji/django-cms,netzkolchose/django-cms,nimbis/django-cms,jproffitt/django-cms,mkoistinen/django-cms,vxsx/django-cms