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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines