[TextFields] Add pre-verification hook for snapshot tests. (#6131)

Adds a new pre-verification hook for test classes to implement if they need to
perform any additional configuration or customization before the snapshot is
generated. For example, RTL tests need to be able to change the base writing
direction of any `input` text after it's set but before the snapshot image is
captured.

Part of #5762
Prework for #6122