Why do Active Storage tests hang?

I have installed this gem to validate and test active storage:

gem 'active_storage_validations'
gem 'shoulda-matchers', '~> 6.0'

# app/models/asset.rb
class Asset < ApplicationRecord
  has_one_attached :portrait

  validates(
    :name,
    presence: true,
    uniqueness: true,
    format: {
      with: /\A[\w\.\-\(\)\+]+\z/s,
      message: 'Only alphanumeric, and _-()+. characters are allowed.' # rubocop:disable Rails/I18nLocaleTexts
    }
  )

  validates(
    :portrait,
    content_type: ['image/png', 'image/jpeg', 'image/svg+xml'],
    dimension: { width: { in: 8..512 }, height: { in: 8..512 } },
    size: { less_than: 250.kilobytes }
  )
end

# spec/models/asset_spec.rb
require 'rails_helper'

RSpec.describe Asset do
  subject(:asset) { build(:asset) }

  it { is_expected.to validate_uniqueness_of :name }
  it { is_expected.to allow_value('123-45+a_').for(:name) }
  it { is_expected.not_to allow_value('123=').for(:name) }

  it { is_expected.to validate_size_of(:portrait).less_than(250.kilobytes) }
end

# spec/support/active_storage.rb required by spec/rails_helper.rb
require 'active_storage_validations/matchers'
require 'ostruct'

RSpec.configure do |config|
  config.include ActiveStorageValidations::Matchers
end

# spec/support/shoulda_matchers.rb required by spec/rails_helper.rb
Shoulda::Matchers.configure do |config|
  config.integrate do |with|
    with.test_framework :rspec
    with.library :rails
  end
end

# spec/factories/assets.rb
FactoryBot.define do
  factory :asset do
    name { "#{Forgery::Internet.user_name}#{Asset.count}" }
    portrait { nil }
  end
end

When I run all tests together everything seems to be working well, but when I run only model specs, it hangs! Why?! 😳

$ rspec spec/models
.....

It is a matter of correct configuration of Active Job:

# config/environments/test.rb
Rails.application.configure do
  # Use inline job processing to make things happen immediately
  config.active_job.queue_adapter = :inline
endd

I found the answer here: https://stackoverflow.com/a/62310984/7896107