Bump rubocop-minitest from 0.29.0 to 0.31.0

Bumps [rubocop-minitest](https://github.com/rubocop/rubocop-minitest) from 0.29.0 to 0.31.0.
- [Release notes](https://github.com/rubocop/rubocop-minitest/releases)
- [Changelog](https://github.com/rubocop/rubocop-minitest/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-minitest@v0.29.0...v0.31.0)

---
updated-dependencies:
- dependency-name: rubocop-minitest
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -49,7 +49,7 @@ GEM
       unicode-display_width (>= 2.4.0, < 3.0)
     rubocop-ast (1.28.1)
       parser (>= 3.2.1.0)
-    rubocop-minitest (0.29.0)
+    rubocop-minitest (0.31.0)
       rubocop (>= 1.39, < 2.0)
     rubocop-performance (1.17.1)
       rubocop (>= 1.7.0, < 2.0)