TST: Add Series.update ExtensionArray tests
cc @simonjayhawkins 

xref https://github.com/pandas-dev/pandas/issues/25744 https://github.com/pandas-dev/pandas/issues/34020

I think the dtype issue for DataFrame.update with categorical columns mentioned in the first issue is still unaddressed, so likely don't want to only close and lose that (perhaps break into its own issue)?