test,net: add tests for server.connections

There were no tests confirming situations where server.connections
should return `null`. Add a test for that situation.

Expand existing server.connection test slightly to check value.

Refactor (mostly spacing) code for server.connections setter.

PR-URL: https://github.com/nodejs/node/pull/10762


Reviewed-By: James M Snell <jasnell@gmail.com>