require 'active_model/cases/helper'

module ClientSideValidations
  class ActiveModelTestBase < ::ActiveModel::TestCase
    include ::ActiveModel::Validations

    def setup
      @person = Person.new
    end
  end
end

require 'active_model/cases/helper'

module ClientSideValidations
  class ActiveModelTestBase < ::ActiveSupport::TestCase
    include ::ActiveModel::Validations

    def setup
      @person = Person.new
    end
  end
end

Use ActiveSupport::TestCase instead of ActiveModel
Use ActiveSupport::TestCase instead of ActiveModel

The unused ActiveModel::TestCase class was removed in Rails 5.1.0.beta1

Ref: ccfa785535d912c3529b0562d9f2e7a2dc4b7ed7@4f8d86c822b5ceebbd1616f3329fbdf40d3f8431