Update tests to run with network partitioning enabled

There are still a number of tests which have network partitioning explicitly disabled. We should update them to have better test coverage. This will also allow us to remove the network partitioning prefs, see Bug 1756599.

Here is an overview of the affected tests:

• browser/components/originattributes/test/browser/browser_cache.js
• browser/components/originattributes/test/browser/browser_clientAuth.js
• browser/components/originattributes/test/browser/browser_httpauth.js
• browser/components/originattributes/test/browser/browser_imageCacheIsolation.js
• browser/components/privatebrowsing/test/browser/browser_privatebrowsing_cache.js
• All of dom/media/webrtc/tests/mochitests
• netwerk/test/browser/browser_about_cache.js
• toolkit/components/antitracking/test/browser/browser_subResources.js
• toolkit/components/extensions/test/mochitest/test_ext_identity.html

There are a few other tests which test with network partitioning enabled and disabled. For these we can simply removed the "disabled" branches.