Fix db state leaking across tests

[bf4]

See e.g. https://travis-ci.org/rails-api/active_model_serializers/builds/97975719

db records created specs weren't being cleared, so that
other specs calling all had unexpected records in them.

TODO: include a mixin to automatically clear records. Maybe

module TransactionalTests
   # Minitest.run_one_method isn't present in minitest 4
   if $minitest_version > 4 # rubocop:disable Style/GlobalVars
     def run_one_method(*)
       super
     ensure
       ActiveRecord::Base.descendants.each do |record_class|
         record_class.delete_all
       end
     end
  end
end
module SomeTest < Minitest::Test
  extend TransactionalTests
end

Perhaps consider replacing all usages of Minitest::Test with ActiveSupport::Test
for better compatibility.

[bf4]

The lack of passing builds on master is distracting to active PRs. So, I'm going to merge this.

[beauby]

Raising my voice a bit too late about this, but this way of handling stuff does not seem right at all. We should find a better solution.

[bf4]

💯

[bf4]

@beauby Yeah, I would have waited longer, but it did fix the immediate issue, and allowed builds to pass, which was blocking good PRs. I'm not how we should handle 'this PR needs a followup'. Maybe a label? Maybe an issue that lists issues? Maybe a new issue?

[beauby]

Not sure either. A label is probably good.

[bf4]

Between that and forgetting to use milestones.. I could use a page that says 'start looking for priority work here and here's some stuff not to forget about', or something like that... rails-bot does some cool stuff.. also auto-marks as stale