Use non-global root expressions for each test

[tchssk]

Related to goadesign/goa#3721

[Copilot]

Pull Request Overview

This PR refactors test files to use non-global root expressions by capturing the return value from httpcodegen.RunHTTPDSL instead of relying on the global expr.Root variable. This change improves test isolation and prevents potential issues with shared state between tests.

Key changes:

• Replace global expr.Root usage with local root variables returned from httpcodegen.RunHTTPDSL
• Update function calls to use the local root variable instead of the global one
• Remove unused expr package imports where no longer needed

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
i18n/generate_test.go	Updates two test functions to use local root variables and removes dependency on global eval context
goakit/mount_files_test.go	Replaces global expr.Root with local root variable and removes unused import
goakit/generate_test.go	Updates three test functions to use local root variables and removes unused import
goakit/encode_decode_files_test.go	Updates two test functions to use local root variables and removes unused import
cors/generate_test.go	Updates test function to use local root variable and removes unused import

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[raphael]

this is great, thank you!