tests: use global Selenium singleton
tests: use global Selenium singleton

By using a global Firefox instance the tests run faster:

from `50,26s user 4,07s system 94% cpu 57,538`
to   `30,74s user 2,08s system 85% cpu 38,370`

(using `tox -e py34-django18 -- -x -s`)

This could be achieved using pytest-splinter, too - but since the tests
have not been converted to pytest yet, I've skipped this for now.

luzfcb/django-autocomplete-light,yourlabs/django-autocomplete-light,yourlabs/django-autocomplete-light,luzfcb/django-autocomplete-light,shubhamdipt/django-autocomplete-light,Perkville/django-autocomplete-light,shubhamdipt/django-autocomplete-light,Visgean/django-autocomplete-light,dsanders11/django-autocomplete-light,dsanders11/django-autocomplete-light,luzfcb/django-autocomplete-light,Visgean/django-autocomplete-light,Perkville/django-autocomplete-light,Visgean/django-autocomplete-light,Eraldo/django-autocomplete-light,shubhamdipt/django-autocomplete-light,yourlabs/django-autocomplete-light,Perkville/django-autocomplete-light,Visgean/django-autocomplete-light,shubhamdipt/django-autocomplete-light,Eraldo/django-autocomplete-light,Eraldo/django-autocomplete-light,dsanders11/django-autocomplete-light,Perkville/django-autocomplete-light,dsanders11/django-autocomplete-light,yourlabs/django-autocomplete-light,Eraldo/django-autocomplete-light,luzfcb/django-autocomplete-light