Bump test-unit from 3.6.7 to 3.6.8

Bumps [test-unit](https://github.com/test-unit/test-unit) from 3.6.7 to 3.6.8.
- [Release notes](https://github.com/test-unit/test-unit/releases)
- [Commits](test-unit/test-unit@3.6.7...3.6.8)

---
updated-dependencies:
- dependency-name: test-unit
  dependency-version: 3.6.8
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -3,14 +3,14 @@ GEM
   specs:
     kramdown (2.5.1)
       rexml (>= 3.3.9)
-    power_assert (2.0.4)
+    power_assert (2.0.5)
     prettier_print (1.2.1)
     rake (13.2.1)
     rexml (3.3.9)
     rouge (4.5.1)
     syntax_tree (6.2.0)
       prettier_print (>= 1.2.0)
-    test-unit (3.6.7)
+    test-unit (3.6.8)
       power_assert
 
 PLATFORMS