Adding Back Rails::Engine::Railties#engines

Removing it breaks functionality with gems such as Thinking Sphinx.
This restores it with a deprecation warning. Closes #8551
Missing repair_validations
Reset table_name_{prefix,suffix}, and table_name after each test

because some tests were not resetting them, and thus the tests were order dependent
Ignore "SHOW search_path" when counting PostgreSQL queries
Be sure to clear schema cache
Ignore schema queries on SQLite 3 as well
Preload some join table schemas
Make sure to reset default_url_options
Capture block so content won't leak.

The [following pull request](https://github.com/rails/rails/pull/8916) fixed
the block being passed to the appropriate helper method. However, the content
being passed into the block is generating repeated markup on the page due to
some weird ERb evaluation.

This commit tries to capture the block's generated output so the page isn't
flooded with markup.

[Rafael França + José Mota]

Closes #8936
Stylistic pass at form_helper_test
Restore I18n.locale after running tests
Reset SchemaMigration after updating
Just ignore all PRAGMA queries
Merge branch 'isolating_tests'

Now we're almost ready to remove this: https://github.com/rails/rails/blob/5294ad8/activesupport/lib/active_support/test_case.rb#L29
Merge pull request #8557 from timraymond/add_back_engines_and_deprecate

Adding Back Rails::Engine::Railties#engines

Conflicts:
	railties/CHANGELOG.md
Whitespaces :scissors:

[ci skip]
Remove tabs, use spaces :scissors:

[ci skip]
Fix images in getting started guide.

We changed the error style, so they're all wrong. Now they're right.
:smile:
Getting Started Guide: strong_parameters

Stuff didn't work because of strong_parameters. Now the right calls are
in place.
fixing link to validations guide in getting started guide