test: fix flaky test-cluster-send-handle-twice

Use `common.mustCall()` to make sure connection callback runs exactly
once.

Use `connect` event instead of `setTimeout` to avoid test failing if
timer runs before client is connected.

Remove `cluster.worker.disconnect()` after `assert.fail()`. It is
unreachable code that is unnecessary.

PR-URL: https://github.com/nodejs/node/pull/19700


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>