[Fix rubocop#73] The Minitest department works on _test.rb by default

Fixes rubocop#73.

This PR changes the Minitest department works on file names end with
`_test.rb` by default.

RuboCop Minitest only targets files in `test` directory by default until v0.8.1.

```yaml
Minitest:
  Enabled: true
  Include:
    - '**/test/**/*'
```

https://github.com/rubocop-hq/rubocop-minitest/blob/v0.8.1/config/default.yml

But test files may be located in a directory other than `test` directory.
This PR adds files that end of filename is `_test.rb` to be considered test files.

This behavior is similar to how RuboCop RSpec works on `_spec.rb`.
https://github.com/rubocop-hq/rubocop-rspec/blob/v1.38.1/config/default.yml#L5

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Changes
+
+* [#73](https://github.com/rubocop-hq/rubocop-minitest/issues/73): The Minitest department works on file names end with `_test.rb` by default. ([@koic][])
+
 ## 0.8.1 (2020-04-06)
 
 ### Bug fixes

config/default.yml

@@ -2,6 +2,7 @@ Minitest:
   Enabled: true
   Include:
     - '**/test/**/*'
+    - '**/*_test.rb'
 
 Minitest/AssertEmpty:
   Description: 'This cop enforces the test to use `assert_empty` instead of using `assert(object.empty?)`.'