Add mock hook implementation to mw mock
- This allows us to call the mw.hook function in our components without causing an error in the test suite.
Bug: T407278
How to register an account on GitLab. Due to spam, new accounts are locked until approved by an admin or the approver bot. Your GitLab account gets automatically approved within one hour if you are a member of Trusted Contributors in Gerrit, or a member of the Trusted-Contributors group in Phabricator and linked your Developer account ("LDAP") to your Phabricator account. If none of these apply, file an unlock request to expedite access. Only if you are WMF staff: Create your GitLab account, have "Wmf" group membership in IDM, link your Developer account ("LDAP") to your Phabricator account, and get automatically approved.
Support: mw:GitLab, how to host a project on GitLab, #wikimedia-gitlab on libera.chat, #GitLab on Phabricator.
Bug: T407278