It looks like test/unit/tracker_test.rb has an unspecified dependency on the :enabled_modules fixture. To reproduce:

bundle exec rake db:test:prepare
bundle exec rake test TEST=test/unit/tracker_test.rb

=>
TrackerTest#test_visible_scope_chained_with_project_rolled_up_trackers [/home/circleci/tablexi/xi-redmine/test/unit/tracker_test.rb:40]:
Expected: [2]
  Actual: []

Environment:
  Redmine version                3.4.6.devel
  Ruby version                   2.4.1-p111 (2017-03-22) [x86_64-darwin17]
  Rails version                  5.2.1
  Environment                    production
  Database adapter               Mysql2
  Mailer queue                   ActiveJob::QueueAdapters::AsyncAdapter
  Mailer delivery                smtp
Redmine plugins:
  no plugin installed

The spec passes when the :enabled_modules fixture is specified.