chore(browserstack): properly teardown tunnel

[devversion]

• Implements the browserstack teardown script.

[jelbourn]

Is it worth capping how long we wait here?

[devversion]

I think it just helps ensuring that the Browserstack tunnel is properly turned off.

Normally the while does not run once, because the process terminated synchronously before, but in some situations it might take longer (crashes etc.) and this might help us as well.

Also added because the Saucelabs teardown script had the same.

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