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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test net