Enable RuboCop Minitest and RuboCop Rake for development

Based on the following information, `rubocop-minitest` and `rubocop-rake` have been added
for development. Autocorrection has been applied for offenses detected by `rubocop-minitest`.

```console
$ bundle exec rake
(snip)

Running RuboCop...
Inspecting 40 files
........................................

40 files inspected, no offenses detected

Tip: Based on detected gems, the following RuboCop extension libraries might be helpful:
* rubocop-minitest (https://rubygems.org/gems/rubocop-minitest)
* rubocop-rake (https://rubygems.org/gems/rubocop-rake)

You can opt out of this message by adding the following to your config
(see https://docs.rubocop.org/rubocop/extensions.html#extension-suggestions for more options):
AllCops:
    SuggestExtensions: false
```

If preserving the existing behavior is preferred, it is also possible to set `SuggestExtensions: false`.

.rubocop.yml

@@ -1,5 +1,10 @@
 inherit_gem:
   rubocop-shopify: rubocop.yml
+
+plugins:
+  - rubocop-minitest
+  - rubocop-rake
+
 Naming/FileName:
   Exclude:
     - 'lib/mcp-ruby.rb'