chore(tests): Added variant flag

[github-actions]

Note

The pull request "chore(tests): Added variant flag" was created by @logaretm but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

I have been running a lot of Next.js tests and needed to run specific variants against my changes, so I made these changes to our test runner script and thought to share it with everyone.

[linear]

JS-1286

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #18458, which was included in the 10.31.0 release.