Fix for rails 8.1

[cub8]

Motivation

Rails 8.1 updated class executable attribute definition in Rails::TestUnitReporter:

# previously
class_attribute :executable, default: ENV.fetch("RAILS_TEST_EXECUTABLE", "bin/rails test")

# currently
singleton_class.attr_accessor :executable

As a consequence, when tests failed there would be a nasty error because Shoulda used executable accessor instead of self.class.executable.

Fix

• Changed executable call to self.class.executable. This works because class_attribute :executable allows both self.class.executable call on singleton class and executable on class instance
• Added rails 8.1 Appraisal and tests
• adjusted tests code to work with older Rails versions (6.1, 7.0) and Rails 8.1

[cub8]

Just realized that first, it's the case with shoulda-matchers and not shoulda and there's already way around this problem: thoughtbot/shoulda-context#109

Sorry for causing chaos 😅