Bump rack from 2.2.6.4 to 2.2.8.1 in /tests

Bumps [rack](https://github.com/rack/rack) from 2.2.6.4 to 2.2.8.1.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.6.4...v2.2.8.1)

---
updated-dependencies:
- dependency-name: rack
  dependency-type: direct:production
...

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

tests/Gemfile.lock

@@ -24,7 +24,7 @@ GEM
       racc (~> 1.4)
     public_suffix (4.0.6)
     racc (1.6.2)
-    rack (2.2.6.4)
+    rack (2.2.8.1)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
     rake (13.0.6)