Closes #9325.

I adjusted the example and the description in the testing guide and
simply linked from the mailer to the testing guide. This way we don't
have to maintain two separate places.

The env hash passed to `Http::Headers#new` must be in env format.
Also be aware that the passed hash is modified directly.

docs and test-cases for setting headers/env in functional tests.

Follow up to #9700.

Any flags that got set will be passed through to MiniTest::Unit.runner,
such as `-n`, `-s-, and `-v`.

* Update test invocation to use `rails test` instead.
* Update all the test command previews (since we're now using MiniTest.)
* Mentioning MiniTest instead of Test::Unit.
* Update list of test suites.

Update documentation for yml usage against https://github.com/rails/rails/issues/8612

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

Talked with @fxn about this. Bullet points should have periods at the ends.

The proper name, as per the documentation guides, is 'Active Record.' I've
updated several references to it as 'ActiveRecord'.