Update mighty_test requirement from ~> 0.3.0 to ~> 0.4.0

Updates the requirements on [mighty_test](https://github.com/mattbrictson/mighty_test) to permit the latest version.
- [Release notes](https://github.com/mattbrictson/mighty_test/releases)
- [Commits](mattbrictson/mighty_test@v0.3.0...v0.4.0)

---
updated-dependencies:
- dependency-name: mighty_test
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile

@@ -4,7 +4,7 @@ source "https://rubygems.org"
 gemspec
 
 gem "concurrent-ruby", "~> 1.1"
-gem "mighty_test", "~> 0.3.0" unless RUBY_VERSION < "3.1"
+gem "mighty_test", "~> 0.4.0" unless RUBY_VERSION < "3.1"
 gem "minitest", "~> 5.11"
 gem "rake", "~> 13.0"
 gem "rubocop", "1.65.1"