[MOVEONLY] [tests] Move addrman ser/deser tests to addrman_tests.cpp

Summary:

Addrman serialization/deserialization tests are currently in net_tests.cpp.
Move them to addrman_tests.cpp with the rest of the addrman tests.

Backport of core#22725.

Depends on D12272.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12274