Bump rack from 3.0.16 to 3.1.18 in /tests

Bumps [rack](https://github.com/rack/rack) from 3.0.16 to 3.1.18.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v3.0.16...v3.1.18)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 3.1.18
  dependency-type: direct:production
...

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

Author: dependabot[bot]

tests/Gemfile.lock

@@ -28,7 +28,7 @@ GEM
       racc (~> 1.4)
     public_suffix (5.0.4)
     racc (1.8.1)
-    rack (3.0.16)
+    rack (3.1.18)
     rack-test (2.1.0)
       rack (>= 1.3)
     rake (13.0.6)