pylint
Refactor: Split out PreviewSync into a separate module.
Fix: Remove unneeded conditionals which ran code only if TRE was available. Now,
       the constructor/destructor contain the only needed conditionals, which
       disable the class as a whole if TRE isn't available.
Refactor: Split test_preview_sync out from test_preview.
Fix: Skip instead of failing tests which need missing prerequisites.