tests: add a test for the 'servicesnames' RPC field

Summary:
This adds a test for both commands in the first commit and renames the test for getnetworkinfo.
Depends on D7948

This is a backport of Core PR16850

Test Plan: ninja && test/functional/test_runner.py rpc_net.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7949