Attempt to make signed_commits_spec.rb less flaky

The examples in this spec often attempt to click a popover before the JavaScript handler is initialized. Since we don't have a good way to detect when initialization has completed, we wait for all requests to finish in an attempt to give the page more time to load before attempting to click.

This won't guarantee that it solves the problem--we'd have to have some indicator like a newly-added class--but hopefully this will work around the problem for now.

Closes #216530 (closed)