[SG-31708] Migrate regression tests from mocha to jest

Created by: gitstart-sourcegraph

Description

This Pull Request is about migrating the regression tests from mocha to jest within client/web/src/regression directory

Test plan

• Check README file

Ref

SG Issue Gitstart Task

Success Criteria

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

Note

• 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!