[macOS] Improve TextInputPlugin test readability (#39664)

This just extracts arguments to the TextInputClient.setClient async
method call into an NSDictionary local to reduce the deep
indentation of the formatted code.

While the bulk of these calls are identical and could be extracted to a
utility function, or a class extension method like
`handleDefaultSetClientCall`, the point of these tests is to exercise
FlutterTextInputPlugin, and the message protocol over the channel are
precisely what's  being tested, so it makes sense to keep these
explicit.

Followup patch to #39632.

Issue: flutter/flutter#120252

Author: cbracken