Add testid to login page

What does this MR do and why?

This change was originally part of !135327 (merged) which also removed the corresponding data-qa-selector . There was an incident due to which we reverted that MR. The e2e tests had failed on staging-canary as the deployed code did not have the UI changes but the test code was using the new selectors. It is doubted that the UI was served from main whereas the tests were using the new code and were expecting canary. A validation was added to catch any future out-of-sync issues. However, the validation is done only after the user logs in so we have to be careful about the login page and remove the data-qa-selector only after the changes here are fully rolled out.

This MR adds data-testid='login-page' selector while keeping the corresponding data-qa-selector around until the new selector reaches both main and canary on live environments. Once done, we can revert the revert.

Related to gitlab-org/quality/quality-engineering/team-tasks#1823 (closed) and gitlab-org/quality/quality-engineering/team-tasks#1822 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.