Update Minitest references in Testing Guide

The Guide references `test/unit`, which has been replaced by `Minitest` in
all versions of Ruby supported by Rails. The following updates were
performed:

* The superclass of `ActiveSupport::TestCase` is no longer
  `MiniTest::Unit::TestCase`, but `Minitest::Test` [1].

* The preferred spelling is `Minitest`, not `MiniTest` [2].

* For a method to be a test in Minitest, its name must start with `test_`,
  not only `test` [3].

* Explanations about `test/unit` in Ruby 1.8 were removed in favor of more
  up-to-date information on `Minitest`.

[1]: https://github.com/rails/rails/blob/6c2b569660c9216595d02ce6412e3bdcc7293317/activesupport/lib/active_support/test_case.rb#L18
[2]: https://github.com/seattlerb/minitest/blob/d5d43cef9a3fd4a0eea972dde125ed5ba1ddb821/lib/minitest.rb#L9
[3]: https://github.com/seattlerb/minitest/blob/d5d43cef9a3fd4a0eea972dde125ed5ba1ddb821/lib/minitest/test.rb#L62
Merge pull request #15425 from leafac/fix-testing-docs

Update Minitest references in Testing Guide