Only add debugger/byebug if on MRI

If the test is interrupted in a way that the teardown block fails to
run, the tests will fail to run until the table is removed manually
without this option.

PG doesn't register it's types using the `int(4)` format that others do.
As such, if we alias `int8` to the other integer types, the range
information is lost. This is fixed by simply registering it separately.

The other option (which I specifically chose to avoid) is to pass the
information of the original type that was being aliased as an argument.
I'd rather avoid that, since an alias should truly be treated the same.
If we need different behavior for a different type, we should explicitly
register it with that, and not have a conditional based on aliasing.

Fixes #18144

[Sean Griffin & ysbaddaden]

This is a doc follow-up to #17739.

/cc @zzak

Fixes #18122

Fix inaccurate docs in active_model errors [ci skip]

Add note about Ajax and CSRF-Token [ci skip]

Clarify ActiveRecord testing guidelines [ci skip]

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

[ci skip]

[ci skip]

The default value for the argument `message` in
`ActiveModel::Errors#add` has a new behavior
since ca99ab24.

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

After
  person.errors.add(:name, nil)
  # => [nil]

Fix typo in nodoc [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

Remove extra bullet in README.md [ci skip]

Validation test fixes

Add 'require' for Hash#with_indifferent_access to active_job/arguments.rb

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.

Testing guide grammar fix

[skip ci]

ActiveJob::Arguments uses Hash#with_indifferent_access.
But, activejob gem does not require Hash extension library.
When we use activejob as standalone,
we need extra require statement. This is unhandy.

This commit fixes it.