Add semaphore to selenium tests
Add semaphore to selenium tests

This change prevents thread pool starvation when running a bunch of
selenium-based tests, by turning the blocking wait for a WebDriver to
start into an async wait.

This also seems to help with speed, and reliability since we're not
running too many browsers at once. I was experencing timeouts, and
seeing them in the debugger while running tests locally, this no longer
happens.

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