[avalanche] Use constants for delays in abc_p2p_getavaaddr

Summary:
The test uses a lot of mock times and it's getting difficult to know who is who, so use constants instead of magic number to make this more obvious. There is no change in behavior.

Depends on D11292.

Test Plan:

./test/functional/test_runner.py abc_p2p_getavaaddr

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11293