Add Rack::Lint to DebugExceptions missing test

[akhilgkrishnan]

Motivation / Background

#48837 missed adding Rack::Lint spec to one of the tests in DebugExceptionsTest.

Detail

This PR adds the Rack::Lint spec for one of tests that was missed in #48837

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[skipkayhil]

Thanks! Linking to #48874 to keep everything organized