Fix flaky features/protected_tags_spec

What does this MR do and why?

Related to #500627 (closed)

This fixes a number of issues with a flaky features/protected_tags_spec spec.

• Wait for dropdowns to become enabled/visible before finding elements within them
• Wait for page.all to find the expected number of elements before returning
• More explicitly select elements

In case this doesn't resolve all of the issues with this spec, and someone comes back to this MR for context, here's a summary of my debugging and investigation: #500627 (comment 2254394276)

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Before	After

How to set up and validate locally

• Checkout this branch
• Run bundle exec rspec --color spec/features/protected_tags_spec.rb as many times as you have time for without failures.

Numbered steps to set up and validate the change are strongly suggested.