Using the string version of the class reference is now deprecated when
referencing middleware. This should be written as a class not as a string.
Deprecation warning that this change fixes:
```
DEPRECATION WARNING: Passing strings or symbols to the middleware
builder is deprecated, please change
them to actual class references. For example:
"ActionDispatch::ShowExceptions" => ActionDispatch::ShowExceptions
```
abstract_unit.rb requires `action_controller` which [already includes the following lines of code](64fcdce1d3/actionpack/lib/action_controller.rb (L2L3)):
```ruby
require 'abstract_controller'
require 'action_dispatch'
```
Goals:
1. Default to :random for newly generated applications
2. Default to :sorted for existing applications with a warning
3. Only show the warning once
4. Only show the warning if the app actually uses AS::TestCase
Fixes#16769
Calling ActiveSupport::TestCase.i_suck_and_my_tests_are_order_dependent! in AS::TestCase makes
everyone's tests order dependent, which should never be done by the framework.
We are planning to remove mocha from our test suite because of
performance problems. To make this possible we should stop require mocha
on ActionSupport::TestCase.
This should not affect applications since users still need to add mocha
to Gemfile and this already load mocha.
Added FIXME notes to place that still need mocha removal
