Generate EE-specific JavaScript fixtures into ee/ directory

This is a follow-up of https://gitlab.com/gitlab-org/gitlab-ee/issues/10386 and a requirement for https://gitlab.com/gitlab-org/gitlab-ce/issues/49127.

Currently fixtures in ee/spec/javascripts/fixtures/ output to spec/javascripts/fixtures/. We need to change that to ee/spec/javascripts/fixtures/. This requires updating the Rake task (karma:fixtures) and also Karma and Jest configuration.