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What	Shortcut
Run all the test	⌘ U
Run 1 test	ctrl ⌥ ⌘ U
Re-run last	ctrl ⌥ ⌘ G

Note: Run 1 test will run the test that your cursor is on.

Of the 3, my favorite got to be re-run last tests ctrl ⌥ ⌘ G, because I re-run tests a lot while fixing things.