Bump minitest from 5.11.3 to 5.12.0

Bumps [minitest](https://github.com/seattlerb/minitest) from 5.11.3 to 5.12.0.
- [Release notes](https://github.com/seattlerb/minitest/releases)
- [Changelog](https://github.com/seattlerb/minitest/blob/master/History.rdoc)
- [Commits](https://github.com/seattlerb/minitest/compare/v5.11.3...v5.12.0)

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

Gemfile.lock

@@ -6,7 +6,7 @@ GEM
     mina (1.2.3)
       open4 (~> 1.3.4)
       rake
-    minitest (5.11.3)
+    minitest (5.12.0)
     mustermann (1.0.3)
     open4 (1.3.4)
     rack (2.0.7)