`ColumnDumper#migration_keys` was extracted to customize keys for
standardized column arguments widths. But the feature was removed in
df84e986. The internal method is no longer used for that.

Remove `ActiveModel::TestCase` from lib

`ActiveModel::TestCase` is used only for the test of Active Model.
Also, it is a private API and can not be used in applications.
Therefore, it is not necessary to include it in lib.

Correct spelling

Add test for DateTime#blank?

Indicate action that failed in YamlColumn

```
go get -u github.com/client9/misspell/cmd/misspell
misspell  -w -error -source=text .
```

Add `:args` to `process.action_mailer` event.

Collect all file patterns when running multiple rake test tasks

Docs: Correction: Module::DelegationError

Replaces the rake_patterns instance variable with simple require, as
`autorun` will run tests from all eagerly required test files.

Fixes #27801

When the delegation target is nil and the allow_nil option is not
in use, a Module::DelegationError is raised.

    class C
      delegate :a, to: :b
        def b
          nil
        end
      end
    C.new.a
    # => Module::DelegationError: C#a delegated to b.a, but b is nil

[ci skip]

Fix typo in guide: _articles should be _article

The guide contains a typo in the "local_assigns" section, where
rendering a model named 'Article' via `render @articles` is shown to
render a partial called `_articles.html.erb`, when in fact the necessary
partial name is `_article.html.erb`

Fix collection_singular_ids= bug

Fix a test of AR::Type::TypeMap#lookup when using Oracle

Ignore yarn error log file

When the association's primary key is manually set as a symbol and called with an array of strings, CollectionAssociation#ids_writer
fails to cast the ids to integers. This is because
AssociationReflection#association_primary_key_type and
ThroughReflection#association_primary_key_type return the incorrect
type, since ModelSchema.type_for_attribute only accepts a string. The
result is an ActiveRecord::RecordNotFound error.

generate mailer layout files only when it does not exist

Fix CI failure caused by aa647b46

sub_template_message distracts Rails.root from its message only when Rails.root is defined,
and Rails.root is defined at tools/test.rb

since the test names become Regexp filters, non-escaped test names cause RegexpError on isolated test via bin/test

This causes TypeError when loaded separately

If already have layout files, in many cases use it.

According to the commit comment on 54d84cbb, AJ/bin/test was intentionally not added,
but AJ tests doesn't actually do anything special other than specifying ENV['AJ_ADAPTER'],
which can be easily done via command line environment variable.

MySQL generated columns: https://dev.mysql.com/doc/refman/5.7/en/create-table-generated-columns.html
MariaDB virtual columns: https://mariadb.com/kb/en/mariadb/virtual-computed-columns/

Declare virtual columns with `t.virtual name, type: …, as: "expression"`.
Pass `stored: true` to persist the generated value (false by default).

Example:

    create_table :generated_columns do |t|
      t.string  :name
      t.virtual :upper_name, type: :string,  as: "UPPER(name)"
      t.virtual :name_length, type: :integer, as: "LENGTH(name)", stored: true
      t.index :name_length  # May be indexed, too!
    end

Closes #22589

Currently, executing the `test_initialize_raises_an_ArgumentError_if_no_block_given`
test alone will result in an error.

```
$ ./bin/test test/evented_file_update_checker_test.rb -n test_initialize_raises_an_ArgumentError_if_no_block_given
Run options: -n test_initialize_raises_an_ArgumentError_if_no_block_given --seed 6692

# Running:

E

Error:
EventedFileUpdateCheckerTest#test_initialize_raises_an_ArgumentError_if_no_block_given:
NameError: uninitialized constant EventedFileUpdateCheckerTest::Listen
    rails/activesupport/test/evented_file_update_checker_test.rb:21:in `teardown'
```

This is because if do not specify a file or directory for
`EventedFileUpdateChecker`, do not require `listen`, and using listen
method in teardown.
https://github.com/rails/rails/blob/master/activesupport/lib/active_support/evented_file_update_checker.rb#L53..L65

Therefore, added listen's require to avoid errors.

call `gem` `erubis` before require

For inform that need to add `erubis` to gemfile.

Currently, executing the `test_initialize_raises_an_ArgumentError_if_no_block_given`
test alone will result in an error.

```
$ ./bin/test test/evented_file_update_checker_test.rb -n test_initialize_raises_an_ArgumentError_if_no_block_given
Run options: -n test_initialize_raises_an_ArgumentError_if_no_block_given --seed 6692

# Running:

E

Error:
EventedFileUpdateCheckerTest#test_initialize_raises_an_ArgumentError_if_no_block_given:
NameError: uninitialized constant EventedFileUpdateCheckerTest::Listen
    rails/activesupport/test/evented_file_update_checker_test.rb:21:in `teardown'
```

This is because if do not specify a file or directory for
`EventedFileUpdateChecker`, do not require `listen`, and using listen
method in teardown.
https://github.com/rails/rails/blob/master/activesupport/lib/active_support/evented_file_update_checker.rb#L53..L65

Therefore, added listen's require to avoid errors.

remove unused variable

This removes the following warnings.

```
activesupport/test/file_update_checker_shared_tests.rb:279: warning: assigned but unused variable - checker
```

Fix missing partial iteration