Bump test-unit from 3.6.8 to 3.6.9 in /steep

Bumps [test-unit](https://github.com/test-unit/test-unit) from 3.6.8 to 3.6.9.
- [Release notes](https://github.com/test-unit/test-unit/releases)
- [Commits](test-unit/test-unit@3.6.8...3.6.9)

---
updated-dependencies:
- dependency-name: test-unit
  dependency-version: 3.6.9
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

steep/Gemfile.lock

@@ -73,7 +73,7 @@ GEM
     strscan (3.1.5)
     terminal-table (4.0.0)
       unicode-display_width (>= 1.1.1, < 4)
-    test-unit (3.6.8)
+    test-unit (3.7.0)
       power_assert
     tzinfo (2.0.6)
       concurrent-ruby (~> 1.0)