Add a section about job testing [ci skip]

* Cover lower level job testing (assertions about the execution of the
  job itself).
* Add a table with the custom assertions provided by Active Job's
  `TestHelper` module.
* Add an example testing job queuing inside a model
Delegate comparison operator to value
Print out a meaningful error when ActiveRecord::ReadOnlyRecord is raised

Currently, there is no messages which get printed out. Convoluted system
may have hooks that create other objects in which case we only fail with
no messages. This commit changes this information allowing you to know
which object is the one that actually raised the error.
Merge pull request #17541 from OpenSourceProjects/meaninful_message

Print out a meaningful error when ActiveRecord::ReadOnlyRecord is raised
[ci skip] modify default host in command line guide
Merge pull request #17551 from y-yagi/fix_command_line_guide

[ci skip] modify default host in command line guide
docs, the abstract data type `:timestamp` was removed. See #15184 [ci skip]
remove the last traces of OpenBase from our codebase. [ci skip]
pg tests, only execute what's necessary.
pg tests, get rid of `sql_types_test.rb`.
Guides: Corrected name for assigns key [ci skip]

- Integration test example is now using 'articles' name
  for both path and assigns key.
Merge pull request #17552 from jipe/master

Guides: Corrected name for assigns key [ci skip]
default scopes should break the cache on has_many.

if you specify a default scope on a model, it will break caching.  We
cannot predict what will happen inside the scope, so play it safe for
now.  fixes #17495
Merge pull request #17493 from petewest/duration-comparable

Delegate comparison operator to value
[ci skip] Fix line break on command line guide
default scopes should break the cache on singulur_association.

fixes #17495
Merge pull request #17476 from robin850/job-testing

Add a section about job testing [ci skip]
Merge pull request #17559 from alfa-jpn/fix/singular_association_cache

default scopes should break the cache on singulur_association.
Merge pull request #17557 from yui-knk/fix/command_line_guide

[ci skip] Fix line break on command line guide
Documenting Rails::Info module [ci skip]