Add note about Ajax and CSRF-Token [ci skip]
- Fixed unused variable
- Changed test to verify complete message instead of verifying if message contains text.
Clarify ActiveRecord testing guidelines [ci skip]
1. Specify that you need to create the test databases, and that no special
   Rails command needs to be run to do that.
2. Although the underscore style of `rake test_mysql` works, make the
   documentation of running the tests in RUNNING_UNIT_TESTS.rdoc
   consistent with the "Contributing..." guide.
3. Promote "Testing Active Record" to not be a subsection of
   "Running a Single Test," since it doesn't make sense as a subsection
   of that.
Merge pull request #18118 from vipulnsward/fix-validation-message

Validation test fixes
Remove bullet
Merge pull request #18126 from Decagon/patch-1

Remove extra bullet in README.md [ci skip]
Remove unused Sheep fixture from AV

The `Sheep` fixture was added in eb23754e when moving template tests
from actionpack to actionview, but it's not actually used in ActionView tests.

The `Sheep` fixture is only used to test `uncountable` in ActiveModel tests,
and is already defined in activemodel/test/models/sheep.rb
Merge pull request #18129 from claudiob/remove-unused-sheep-fixture-from-action-view

Remove unused Sheep fixture from AV
`bin/rake` does not exist in plugin directory [ci skip]
Fix typo in nodoc
Merge pull request #18132 from rochefort/fix-typo-nodoc

Fix typo in nodoc [ci skip]
Fix inaccurate docs in active_model errors [ci skip]

The default value for the argument `message` in
`ActiveModel::Errors#add` has a new behavior
since ca99ab2481d44d67bc392d0ec1125ff1439e9f94.

Before
  person.errors.add(:name, nil)
  # => ["is invalid"]

After
  person.errors.add(:name, nil)
  # => [nil]
Better explain what `test_helper.rb` is designed for [ci skip]
We may only look at some of the methods available from TestCase [ci skip]
Better explain how test() methods get defined and kill this run-on sentence.

[ci skip]
Removed run-on sentence and replace with sectional comments [ci skip]
Fix NOTE for method naming in Ruby and add intro sentence to next section.

[ci skip]
Remove duplicate description of the previous line [ci skip]
Better explain number of assertions per test [ci skip]
Merge pull request #18131 from y-yagi/fix_command_in_plugins_guide

`bin/rake` does not exist in plugin directory [ci skip]