Introduce Spree::Event's test interface to run only selected listeners

[waiting-for-dev]

Description

Given the global nature of our event bus, we need a system to scope the
execution of a block to a selected list of subscribers. That's useful
for testing purposes, as we need to be sure that others subscribers are
not interfering with our expectations.

This commit introduces a Spree::Event.performing_only(listeners)
method. It takes a block during the execution of which only the provided
listeners are subscribed:

listener1 = Spree::Event.subscribe('foo') { do_something }
listener2 = Spree::Event.subscribe('foo') { do_something_else }

Spree::Event.performing_only(listener1) do
  Spree::Event.fire('foo') # only listener1 will run
end

Spree::Event.fire('foo') # both listener1 & listener2 will run

This behavior is only available for the new
Spree::Event::Adapters::Default adapter.

We only need that for testing purposes, so the method is made available
after calling Spree::Event.enable_test_interface. It prevents the main
Spree::Event API from being bloated and sends a more explicit message
to users.

We also add a Spree::Subscriber#listeners method, which returns the
set of generated listeners for a given subscriber module. It's called
automatically by Spree::Event.performing_only so that users can
directly specify that they only want the listeners for a given
subscriber module to be run. Spree::Subscriber#listeners accepts an
array of event names as arguments in case more fine-grained control is
required.

module EmailSubscriber
  include Spree::Event::Subscriber

  event_action :foo
  event_action :bar

  def foo(_event)
    do_something
  end

  def bar(_event)
    do_something_else
  end
end

Spree::Event.performing_only(EmailSubscriber) do
  Spree::Event.fire('foo') # both foo & bar methods will run
end

Spree::Event.performing_only(EmailSubscriber.listeners('foo')) do
  Spree::Event.fire('foo') # only foo method will run
end

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)
• I have attached screenshots to this PR for visual changes (if needed)

[aldesantis]

@waiting-for-dev looks good! Couple of comments and then it's GTM. ❤️ Approving because it's pretty small stuff.

[waiting-for-dev]

Thanks for your review @aldesantis, I forced a push with the updates to rebase the dependant branches more easily.

[waiting-for-dev]

Moved to solidusio#4204