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.
Remove conditional, since results are always an instance of `ActiveRecord::Result`
Added SanitizeHelper to rails guide docs [ci skip]

Added SanitizeHelper to rails guide docs [ci skip]

Added SanitizeHelper to rails guide docs update [ci skip]
Creating an SSE class to be used with ActionController::Live.
Added Action View in 'rake about' [ci skip]
Add tmp dir in actionview/test/

Missing tmp directory will cause the test to fail on JRuby. will commit
adds tmp directory & 22 TemplateDigestorTests which were failing before
are now green.

Further Discussion on this can be seen at #11743
Fix: documentation for ActiveModel::Errors
Merge pull request #11757 from ryog/fix_document_active_model_errors

Fix: documentation for ActiveModel::Errors [ci skip]
Added data for permit array into scalar value
Merge pull request #11760 from rahul100885/master

Added data for permit array into scalar value

[ci skip]
getting started guide: pass the resource to the URL helper [ci skip]

Passing the actual ID is not idiomatic.
migrations guide: fix and edits [ci skip]
Free result_metadata directly instead of freeing 2nd, redundant call.

`result_metadata` returns a new object each time it is called, so calling `result_metadata.free` is essentially a noop.  Instead call `free` directly on the metadata when we're done with it.
Removed confusing statements about routes in getting started guide [ci skip]

- routes for CRUD already exists as `resources :posts` is in
  `config/routes.rb`
- So we don't have to add any new route in the `config/routes.rb`
- As per #11644, the adding of routes which are already there confuses user, so here its
  changed to referring output of `rake routes`
Create sqlite3 directory if not present

If the `db/` directory is not present on a remote machine it will blow up in unexpected ways with error messages that do not indicate there is a missing directory:

```
SQLite3::CantOpenException: unable to open database file
```

This PR checks to see if a directory exists for the sqlite3 file and if not creates it for you.

This PR is an alternative to #11692 as suggested by @josevalim
Merge pull request #11763 from schneems/schneems/create-sqlite3db

Create sqlite3 directory if not present
Merge pull request #11762 from peterkovacs/patch-2

Free result_metadata directly instead of freeing 2nd, redundant call.
Merge pull request #11666 from prathamesh-sonpatki/doc-fix

Removed confusing statement about adding route for show action in getting started guide [ci skip]
load fixtures from linked folders
Merge pull request #11767 from kassio/master-load-fixtures-from-linked-folder

Load fixtures from linked folders[master]