Include thread-safety rubocop

[alexpark20]

Include thread_safety rubocop as part of Shopify style rubocop. This PR only enables the small subset of the rules as a starting point.

Note that this is part of Falcon on SFR effort (vault project here).

[rafaelfranca]

This should not be enabled

[alexpark20]

Right, we are not enabling it on SFR too. I will disable it 👍

[rafaelfranca]

This cop is disallowing:

class FooTest < ActiveSupport::TestCase
  setup do
    @something = { a: "A" }
  end
end

Which doesn't have any thread safety issue.

We need to fix that in order to enable it.

[alexpark20]

I will not enable it yet. Good check, lets socialize how we can fix it 👍

[sambostock]

The unknown cop issue is fixed, but now we have the issue that rubocop-thread_safety imposes a minimum version requirement for rubocop which is higher than our current minimum (which we test against, so it breaks CI because the dependencies are unsatisfiable).

[sambostock]

Simply bumping it doesn't work either, as it breaks some assumptions made by the tooling that ensures version compatibility. We'll have to fix that too.