Unit tests to confirm content pref service doesn't write to disk in PBM

If we can confirm that the Content Pref Service doesn't write to disk in private browsing mode, then we can drop our patch for legacy/trac#3229 (closed). See the discussion at https://bugzilla.mozilla.org/show_bug.cgi?id=967809

Probably the most secure way to do this is to write unit tests.