Require yaml explicitly on #database_configuration

It is already called inside type_to_sql method.

Refactor test to use DdlHelper.

Follow-Up to https://github.com/rails/rails/pull/14400

This ensures that all tables are removed after each test and thereby
allowing us to run the tests in a random order.

append link to bad code when error type is SyntaxError

Fix tests not unsubscribing from Notifications.

Ensure we are returning either `true` or `false` for `#==`

460eb83d cused `ActiveRecord::Base#==` to sometimes return `nil` in some cases,
this ensures we always return a boolean value. Also fixed a similar problem in
AR reflections.

See https://github.com/rails/rails/blob/master/activesupport/lib/active_support/notifications.rb#L131

Replace trivial regexp with string or include, twice as fast

HashWithIndifferentAccess#update respects #to_hash

Don't load test env by default.

Closes #13241

In particular, `.new`, `#update`, `#merge`, `#replace` all accept
objects which respond to `#to_hash`, even if those objects are not
Hashes directly.

Fix warning for overshadowing XML variable

Pass a base relation to build_default_scope when joining

Swap Timestamp/Callbacks order in ActiveRecord::Base

Adds explanation of :base attribute to errors.add [ci skip]

[ci skip]

AR .persisted? throws SystemStackError for an unsaved model with a
custom primary_key that didn't save due to validation error

custom primary_key that didn't save due to validation error

ActiveRecord/ActiveModel '#validate' alias for 'valid?'

It's unintuitive to call '#valid?' when you want to run validations but
don't care about the return value.

The alias in ActiveRecord isn't strictly necessary (the ActiveModel
alias is still in effect), but it clarifies.

[ci skip]

When the tests are executed in order, the dependency is loaded at the right time.
However this makes it impossible to execute a single test later down the line.
Let's require the dependecy at the beginning to get them working independent of the order.

This resolves the following error:

```
$ ARCONN=postgresql ruby -Itest /Users/senny/Projects/rails/activerecord/test/cases/xml_serialization_test.rb -n test_to_xml
Using postgresql
Run options: -n test_to_xml --seed 51819

E

Finished in 0.081320s, 12.2971 runs/s, 0.0000 assertions/s.

  1) Error:
DatabaseConnectedXmlSerializationTest#test_to_xml:
NameError: uninitialized constant DatabaseConnectedXmlSerializationTest::REXML
    /Users/senny/Projects/rails/activerecord/test/cases/xml_serialization_test.rb:226:in `test_to_xml'
```

/cc @tgxworld fyi

This will keep the test suite passing with older PG installations.

Add a test case for save(validate: false) with invalid foreign key.

It is supposed to raise ActiveRecord::InvalidForeignKey when the
database has a fk constraint.

This method is not being called anywhere in our code and a GitHub search
show it is not being used in any project.