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Partially resolves WEB-628
This PR is minor improvement to the fixture setup for next-script test:
if one of test assertion using `finally` to teardown next instance
(next.destroy) times out, those clause won't be called so next instance
will leak, makes subsequent test fail. PR moves teardown to `afterEach`
hook instead. This relies on the assumption we run test in band
(serial), but internals of nextdevinstance already assumes it via its
own singleton.
dee944bb0f