test: close all connections to prevent server hang

On IBM i calling server.close after the child process closes is not enough to end the server. For some reason the connection still lingers around with the server even though the process was closed. Calling closeAllConnections cleans up the lingering connection. Then calling server.close stops the server from listening for new connections and ends the server. On other platforms, closing the child client process causes the connection to close properly without having to call closeAllConnections. I ran this test case on Linux and AIX.

This fix should help resolve the timeout in the following test case:

• https://ci.nodejs.org/job/node-test-commit-ibmi/nodes=ibmi73-ppc64/1089/testReport/(root)/test/parallel_test_http_pipeline_flood/