Provide a way to disable ExUnit integration for production use

[lackac]

I'm using Wallaby in a production environment to automate filling of a 3rd party web form. It's not ideal, but we needed a solution even while we're working with the 3rd party on an API based integration.

We are deploying to production with releases built by mix release and this doesn't include ExUnit by default which we would like to keep that way. However Wallaby is trying to ensure that the :ex_unit application is started and attempts to create an ExUnit.after_suite/1 hook in Wallaby.SessionStore. This fails currently in a release because ExUnit is not bundled in it.

I didn't see a way to disable ExUnit integration but I think that would be a useful option for similar use cases. There are several ways this could be achieved:

1. a configuration option that explicitly disables ExUnit integration entirely
2. implicitly disable ExUnit integration in Wallaby.SessionStore by making it conditional bases on function_exported?(ExUnit, :after_suite, 1)
3. provide a way for an alternative implementation of Wallaby.SessionStore

I think option 2 would make the most sense in the current state of the project. Option 3 would be overkill for just this use case while the configuration based approach of option 1 is likely unnecessary for that single place of integration that is causing the issue. It's unlikely anyone would use Wallaby.Feature in a production setting where most of the ExUnit integration exists.