[test] updates to unbroadcast test
Summary:
- add () to function to actually disconnect from p2pconn
- extract max interval into a constant
- disconnect at the end of a subtest rather than start of next
This is a backport of Core PR18807 [2/9]
https://github.com/bitcoin/bitcoin/pull/18807/commits/bd093ca15de762fdaf0937a0877d17b0c2bce16e
Test Plan: test/functional/test_runner.py mempool_u*
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D9029