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
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]
Fix comment of SourceAnnotationExtractor#find_in

Commit(810af6f) changed which extensions are taken into account,
so make to match comment of find_in.
And sort extensions to follow the added order.
Merge pull request #17563 from yui-knk/fix/source_annotation_extractor

[ci skip] Fix comment of SourceAnnotationExtractor#find_in
[ci skip] Fix typo in command_line.md