Don't use a PeerManager for the net group limit test

Summary: This is simply unused, we can just stick to the test suite fixture.

Test Plan:

ninja check-bitcoin-net_tests

Reviewers: #bitcoin_abc, bytesofman, PiRK

Reviewed By: #bitcoin_abc, bytesofman, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16198