Ruby on Rails 【テスト】rails test uninitialized constant ActionCable::Connection::TestCase (NameError)

Question

環境

Ruby 2.6.3
Rails 5.2.4.1
初学者
ローカル
Mac

やろうとしていること

＄rails test

エラーが出ています。

posts_controller_test.rb

require 'test_helper'

class PostsControllerTest < ActionDispatch::IntegrationTest

  test "should get home" do
    get static_pages_home_url
    assert_response :success
  end

  test "should get help" do
    get static_pages_help_url
    assert_response :success
  end

  test "should get about" do
    get static_pages_about_url
    assert_response :success
  end

end

エラー内容

rails test
Running via Spring preloader in process 35038
Traceback (most recent call last):
  30: from -e:1:in `<main>'
  29: from /Users/hoge/.rbenv/versions/2.6.3/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
  28: from /Users/hoge/.rbenv/versions/2.6.3/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
  27: from /Users/hoge/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:55:in `load'
  26: from /Users/hoge/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:55:in `load'
  25: from /Users/hoge/hikimy/bin/rails:9:in `<main>'
  24: from /Users/hoge/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:31:in `require'
  23: from /Users/hoge/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:22:in `require_with_bootsnap_lfi'
  22: from /Users/hoge/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/loaded_features_index.rb:92:in `register'
  21: from /Users/hoge/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:23:in `block in require_with_bootsnap_lfi'
  20: from /Users/hoge/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bootsnap-
.
.
.途中割愛（文字数オーバーのため）
.
.
`without_bootsnap_cache'
/Users/hoge/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/bootsnap-1.4.6/lib/bootsnap/load_path_cache/core_ext/active_support.rb:80:in `block in load_missing_constant': uninitialized constant ActionCable::Connection::TestCase (NameError)

試したこと

• Running via Spring preloader in process を調べたが不明

• uninitialized constant ActionCable::Connection::TestCase (NameError)を調べたが不明

• uninitialized constant「定義したクラスを読むことができない」

• springを一度全て止めて見る→効果なし

ps aux | grep spring

kill -9

• kernel_require.rb:72:in `require'を調べて同じようにいくつか試したが解決せず。

仮説

• rails6 から5にバージョンダウンしたから？
• .rbenv/versions/2.6.3/lib/ruby/site_ruby/2.6.0　この辺を削除して再インストールすればいいのか？
• testの書き方が間違っている？

最後に

原因がまったくつかめておりません。
アドバイスお願い申し上げます。