test: make test-http-agent-maxsockets robust

On a slow/busy machine, `test-http-agent-maxsockets` can fail if the
test takes longer than 5 seconds because that is the default value for
`server.keepAliveTimeout`. Disable `keepAliveTimeout` to make the test
robust.

PR-URL: https://github.com/nodejs/node/pull/15192


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>