Bump test-unit from 3.7.3 to 3.7.6

Bumps [test-unit](https://github.com/test-unit/test-unit) from 3.7.3 to 3.7.6.
- [Release notes](https://github.com/test-unit/test-unit/releases)
- [Commits](test-unit/test-unit@3.7.3...3.7.6)

---
updated-dependencies:
- dependency-name: test-unit
  dependency-version: 3.7.6
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -306,7 +306,7 @@ GEM
     sqlite3 (2.9.0-x86_64-linux-gnu)
     sqlite3 (2.9.0-x86_64-linux-musl)
     stringio (3.1.7)
-    test-unit (3.7.3)
+    test-unit (3.7.6)
       power_assert
     thor (1.4.0)
     timeout (0.4.4)