Add `rails test` command to run the test suite

To run the whole test suite:

    $ rails test

To run the test file(s):

    $ rails test test/unit/foo_test.rb [test/unit/bar_test.rb ...]

To run the test suite

    $ rails test [models,helpers,units,controllers,mailers,...]

For more information, see `rails test --help`.

This command will eventually replacing `rake test:*`, and `rake test`
command will actually invoking `rails test` instead.
Update testing documentation

* 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.
Add support for MiniTest flags in TestRunner

Any flags that got set will be passed through to MiniTest::Unit.runner,
such as `-n`, `-s-, and `-v`.
Load fixtures only when running suites, or `-f`

* `rails test -f` will run the test suites with all fixtures loaded
* New application will now generated without `fixtures :all` line
  enabled by default.
Improve wording for rails test command
Update Rake tasks to call `rails test` instead

Also, print out deprecation warning for other rake tasks except
`rake test` and `rake` (default)
Make sure that `rails test` load test in test env
Cleanup tests for unused variables
add an missing assertion to inheritance_test.rb

assertion for
https://github.com/rails/rails/blob/ad624345e54bd20802de67b2b5c9ef29ecf5
d5f4/activerecord/lib/active_record/inheritance.rb#L32
Merge pull request #9658 from tkhr/add-assert-to-test_company_descends_from_active_record

add an missing assertion to inheritance_test.rb
Merge pull request #9657 from vipulnsward/cleanup_tests

Cleanup tests for unused variables
Merge pull request #9080 from sikachu/master-rails-test

Add `rails test` command to run the test suite
Fix test failure introduced in 3ed41e57

I forgot to run the test suit after changing the task name. :bomb:
Merge pull request #9664 from sikachu/master-fix-test_test_failure

Fix test failure introduced in 3ed41e57
debugger does not work on trunk
default the reaping frequency to 10 seconds
Mention that debugging can be slow.

https://github.com/rails/rails/pull/3180\#issuecomment-14705821 has
mention of rendering going from 1-2ms to 4 seconds with this on,
which seems reasonable: debugging is slow.
Make sure that the test case is run under test env

* Unset $RAILS_ENV that got set by abstract_unit to trigger the default.
* split out environment setting since Ruby 1.9.3 doesn't support inline
  ENV setting.
Merge pull request #9666 from sikachu/master-fix-test_test_failure

Make sure that the test case is run under test env
Use platforms instead of conditionals in Gemfile