Remove rails implementation of describe, alias "test" to "it"

Fix Bleak House link in debugging guide. Closes #7002

Follow the consistency defined in dbc43bcc.

Changes introduced in 7404cda9.
Fix railties build.

Add few information on Fedora and CentOS [ci skip]

Show Routes while Debugging RoutingError

this is so we can show route output in the development when we get a routing error. Railties can use features of ActionDispatch, but ActionDispatch should not depend on Railties.

If someone receives a routing error, they likely need to view the routes. Rather than making them visit '/rails/info/routes' or run `rake routes` we can give them that information on the page.

This reverts commit c47a698d.

Reason: Let's revert pending further discussions

Introduced in 75b340d1

A test was failing due to the way that Relation#inspect causes
association proxies to ignore unsaved records added to the association.

This is fixed by simply calling to_a and letting to_a figure out how to
get the records (which, in the case of associations, takes into account
new records).

I think it is acceptable to do this rather than limiting the query at
the database level:

* It's what we've done in all released Rails versions up to this point
* The goal of the limit is to not flood the console with output - this
  is the problem we're targeting, rather than the actual loading of the
  records from the database
* You probably want to do something with those records later anyway,
  otherwise you wouldn't have built a relation for them.

Small refactoring of as_json method

Allow to register database tasks from different adapters

Allow people to register their own flash types.

Engine table name prefix generator fix

Remove unused code.

Added *instance_writer: false* to stored/serialized attributes.

Limit the number of records in Relation#inspect

[ci skip]

Introduce config.action_mailer.default_options=

Allows to easily set :from, :replay_to, etc. options in
config/application.rb using simple syntax:

    config.action_mailer.default_options = { from: "no-replay@example.org" }

Closes #6747

update test_help to config properly turn natural language option

Last versions of Turn don't monkey patch MiniTest to setup
the natural language option. Here is an
[example](https://github.com/TwP/turn/blob/master/try/test_autorun_minitest.rb#L3).

This patches the following behaviour:

    $ rake test:units
    `<top (required)>': undefined method `use_natural_language_case_names='
    for MiniTest::Unit:Class (NoMethodError)