fix: wait for ephemeral server to listen before dispatching request

[aguidrevitch]

Fix #255

When an app is passed to supertest, request(app) starts a temporary server with app.listen(0) and builds the URL from server.address().port. The port is instantly ready before the server emits 'listening', so if superagent dispatches in that window the request misses the app and returns a 404. That's why the route in #255 works from other clients but intermittently 404s under supertest. That's why starting a server outside of the supertest (with waiting for 'listening' event) also fixes the issue.

This PR waits for the server's 'listening' event before dispatching, but only when supertest started the server and it isn't listening yet. A regression test covers the race window, fails without fix, passes with the fix.

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