Improve the testing guide [ci skip]

Improve the "Testing Rails Applications" guide adding a section for
testing helpers and a little note about associations with ActiveRecord
and YAML fixtures.

Also add a link to Fixture Builder ; yet another replacement for
fixtures such as Machinist or FactoryGirl.
Creating an SSE class to be used with ActionController::Live.
Added Action View in 'rake about' [ci skip]
Removing obsolete html tags in favor of markdown code block.
Revert "Removing obsolete html tags in favor of markdown code block"

This reverts commit 157dc275da0ad38635337b7c1d96de656d91de8e.
Minor align BR tags with HTML void - with reference to the generators code change - https://github.com/rails/rails/commit/f038d4cc5fe1ac21f92c7e32c61ff85a0c816f3f
Add tests for ActiveModel::Serializers::JSON#as_json ordering
Avoid Skip in test, have a unified test for order
Rely on NoMethodError#name when deciding to raise DelegationError.

Different Ruby implementations present backtraces differently, as
it should be an information consumed by humans.
A better implementation should use data from the error, in this case
returned by NoMethodError#name.

Fixes issues with Rubinius, which presents backtraces differently from
MRI.
guides should tell people to write ruby code, not output
Merge branch 'master' of github.com:rails/docrails
document `assert_not_XYZ` in place of `refute_XYZ`.
Removed redundant asserts for assigns attribute operation
Merge pull request #11886 from jetthoughts/remove_redundant_assert_from_datatype_test

Removed redundant asserts for assigns attribute operation
Rescue invalid ip address exceptions on assign.

In order that set attribute should not be bang method
Merge pull request #11574 from jetthoughts/11552_rescue_on_invalid_inet_assign

Fix assign ip address with invalid values raise exception
Added docs for 'remove' method [ci skip]
Added new line [ci skip]
Typo Fix[ci skip]
Merge pull request #11887 from raysrashmi/typo_fix

Typo Fix[ci skip]