LPS-49771 Decoupling Bookmarks from export/import tests

Hey Brian,

These changes decoupling the bookmarks from two staging tests: PortletDataContextReferencesTest and PermissionExportImportTest.

Since the PortletDataContextReferencesTest is serializing during the test, I had to create individual mock classes instead of private classes to avoid serializing the container class - the test class itself - as well.

I've also added some minor failsafe tweaks.

The changes were reviewed by @migue

Thanks,

Máté