Bump test-unit from 3.6.2 to 3.6.4 in /gemfiles/truffleruby

Bumps [test-unit](https://github.com/test-unit/test-unit) from 3.6.2 to 3.6.4.
- [Release notes](https://github.com/test-unit/test-unit/releases)
- [Commits](test-unit/test-unit@3.6.2...3.6.4)

---
updated-dependencies:
- dependency-name: test-unit
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

gemfiles/truffleruby/Gemfile.lock

@@ -15,7 +15,7 @@ GEM
     rake (13.2.1)
     rake-compiler (1.2.8)
       rake
-    test-unit (3.6.2)
+    test-unit (3.6.4)
       power_assert
 
 PLATFORMS