Bump minitest from 5.20.0 to 5.21.0

Bumps [minitest](https://github.com/minitest/minitest) from 5.20.0 to 5.21.0.
- [Changelog](https://github.com/minitest/minitest/blob/master/History.rdoc)
- [Commits](minitest/minitest@v5.20.0...v5.21.0)

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

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

Author: dependabot[bot]

Gemfile.lock

@@ -14,7 +14,7 @@ GEM
       temple (>= 0.8.2)
       thor
       tilt
-    minitest (5.20.0)
+    minitest (5.21.0)
     prettier_print (1.2.1)
     rake (13.1.0)
     simplecov (0.22.0)