[cq] Cleanup extract_widget_test

This also removes some unused code in the assist processor test
framework.

Change-Id: I07fce90d2cc099f11a22fbaa66d2ed9a3b589af1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/417883
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

Author: bwilkerson