Write Tests for API Examples of `cupertino_text_field.0`, `data_table.0`, `icon_button.2` & `ink_well.0` (#139258)

Write Tests for API Examples of `cupertino_text_field.0`, `data_table.0`, `icon_button.2` & `ink_well.0`

Note: test for `cupertino_text_field.0` was already there but it was named `cupertino_text_field.0.dart`. I renamed it to `cupertino_text_field.0_test.dart`.

Part of #130459

This PR writes tests for a few of the API examples (not all), as requested in #130459. For the test names, I used the existing tests in the `api` folder as guide.