Added maximize_window() for failing test

Investigated the 1 fail on CI (http://qa-selenium.mv.mozilla.com:8080/view/MozTrap/job/moztrap.dev/48/testReport/tests.test_pinning_filters/TestPinningFilters/test_that_pinning_filters_on_product_and_version_and_suite_set_defaults_in_new_case/)
and the reason why this test fails is because the window needs to be maximized.
Tested this locally and got the same fail without it maximized. (http://qa-selenium.mv.mozilla.com:8080/view/MozTrap/job/moztrap.dev/48/HTML_Report/?)