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:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -54,7 +54,7 @@ GEM
     parser (3.3.7.3)
       ast (~> 2.4.1)
       racc
-    power_assert (2.0.4)
+    power_assert (2.0.5)
     prism (1.4.0)
     proc_to_ast (0.2.0)
       parser
@@ -152,7 +152,7 @@ GEM
     strscan (3.1.2)
     terminal-table (4.0.0)
       unicode-display_width (>= 1.1.1, < 4)
-    test-unit (3.6.7)
+    test-unit (3.6.8)
       power_assert
     tzinfo (2.0.6)
       concurrent-ruby (~> 1.0)