Make use of ADDR_SOFT_CAP outside just the seeder test suite

Summary:
There is already a variable that does this in the node software, but it
cannot be included in the seeder without building the net.h with the
rest of the seeder. Thus expanding the use of ADDR_SOFT_CAP outside
of just the test suite is a better solution.

Test Plan:

ninja
ninja check-bitcoin-seeder

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5682