Motivation / Background
---

While the `ActionView::TestCase` class isn't marked with a `:nodoc:`
comment to indicate that it's internal to Rails, there isn't much
content in the guides that explains how to test view partials.

Libraries like
[view_component](https://github.com/ViewComponent/view_component/) have
[built-in support for
testing](https://viewcomponent.org/guide/testing.html), including
Capybara integration.

While `ActionView::TestCase` already integrates with
`rails-dom-testing`, that integration could be better documented.
Additionally, it wouldn't take much for consuming applications to mimic
the ViewComponent testing experience for their Action View Partials.

Details
---

First, link to the "Testing Rails Applications" page from the
`ActionView::TestCase` class documentation.

Next, add a "Testing View Partials" section to the guides that expands
upon the variety of tooling available to tests that inherit from
`ActionView::TestCase`. In that section, cover topics like:

* the `render` helper method
* the `rendered` helper attribute reader
* calls to `assert_select` with attribute placeholders
* the `document_root_element` helper method
* integration with Ruby's Pattern Matching
* opportunities to integrate with Capybara

Additional Information
---

Additionally, add test coverage that exercise the examples shared in the
new section, including:

* Calls to `assert_select` that utilize attribute placeholders
* Ruby 3.0's Pattern Matching
* Integration with Capybara