Migrate regression tests from `mocha` to `jest`

Created by: umpox

Part of https://github.com/sourcegraph/sourcegraph/issues/31707

Implementation

1. Migrate all tests in client/web/src/regression from Mocha to Jest
2. Ensure tests still run and pass as expected

Notes:

• Due to previous issues, this PR did the opposite migration: https://github.com/sourcegraph/sourcegraph/pull/7872. It could be helpful to look at that PR for an idea of what changes might need to be made.
• Jest has a codemod that could help with this. See these docs: https://jestjs.io/docs/migration-guide#jest-codemods. I tried this briefly but it didn't seem too effective. Use with caution!