Remove test card from empty actor form

[flash1293]

What

This PR removes the "Test" CTA from the empty source/destination form to make it more likely for the user to succeed setting up their connection (related discussion here: #21698 (comment) )

How

• When in edit mode:
  • Always show test card with "Retest" button to retest saved state
  • Disable "Retest" button if the form is dirty
• When in new mode:
  • Only show test card if testing is in progress or was successful/failed
  • Do not show a "Test" button
  • Show a cancel button (disabled if the connection test is not in progress)

https://www.loom.com/share/58ec66cb14d241809881d7480b1bb211

[flash1293]

cc @timroes

[timroes]

Tested locally, works as expected. Code LGTM. Minor nitpick: the error icon can squeeze (not directly related, but easy to fix as part of this PR).

[flash1293]

@nataliekwong could you check out this fix?

[nataliekwong]

@flash1293 Thanks! Looks good to me!