fix caching of default account in forked selenium specs
fix caching of default account in forked selenium specs

test plan:
 * run all specs in spec/selenium/content_migrations_spec.rb serially

Change-Id: Ic23b718b296a8a205eed654a77c37723a8111329
Reviewed-on: https://gerrit.instructure.com/9851
Tested-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@instructure.com>
Reviewed-by: Zach Wily <9de8c4480303b5335cd2a33eefe814615ba3612a@instructure.com>

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