Fix flaky tests in spec/features/groups/issues_spec.rb

What does this MR do and why?

The tests in spec/features/groups/issues_spec.rb attempt to load a list of issues and then drag them to a different order. However, if the dragging attempts to start before they're actually loaded, the JavaScript can fail with an error. To make this test more reliable, wait for the requests to be done before attempting to execute JavaScript.

Relates to #378591 (closed)

How to set up and validate locally

Run the following tests a few time:

bundle exec rspec ./spec/features/groups/issues_spec.rb:173

I was able to reproduce it more frequently with FOSS_ONLY=1 set for both Webpack and bundle exec rspec.

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