I am not very happy about this but we came to the case where it might be useful to explicitly uninstall some hook(s) for out unit tests.
You might want to checkout MediaWikiTestCase::uninstallHook
https://gerrit.wikimedia.org/r/#/c/99349/
I am not happy about blurring differences between unit and integration testing, but breaking core with extensions and vice versa is sometimes useful.
//Saper