[Gecko Bug 1879832] [wdspec] Properly end bidi session when stopping geckodriver in mozilla specific tests

[moz-wptsync-bot]

Differential Revision: https://phabricator.services.mozilla.com/D210329

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1879832 gecko-commit: c10f51249c069391572174bb11b2501903095ef6 gecko-reviewers: webdriver-reviewers, Sasha, whimboo