[tests] Update Unit Test for addrman.h/addrman.cpp

Summary:
Add test for adding multiple addresses to address manager
Clean up unnecessary modulo operations
Add test for GetNewBucket's alternate method signature

This is a backport of Core PR10287

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1861