TST: test custom _formatter for ExtensionArray + revert ExtensionArrayFormatter removal
xref https://github.com/pandas-dev/pandas/pull/26833#issuecomment-502012046

@simonjayhawkins this would be a test for the `_formatter` functionality (it is passing on master before the removal of ExtensionArrayFormatter

Adds an actual test for EA._formatter + reverts https://github.com/pandas-dev/pandas/pull/26833 until we have a more complete solution for simplifying the formatters that take the EA._formatter into account.