Bump test-unit from 3.7.7 to 3.7.8

Bumps [test-unit](https://github.com/test-unit/test-unit) from 3.7.7 to 3.7.8.
- [Release notes](https://github.com/test-unit/test-unit/releases)
- [Commits](test-unit/test-unit@3.7.7...3.7.8)

---
updated-dependencies:
- dependency-name: test-unit
  dependency-version: 3.7.8
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -310,7 +310,7 @@ GEM
     sqlite3 (2.9.4-x86_64-linux-gnu)
     sqlite3 (2.9.4-x86_64-linux-musl)
     stringio (3.2.0)
-    test-unit (3.7.7)
+    test-unit (3.7.8)
       power_assert
     thor (1.5.0)
     timeout (0.6.1)