Add a separate executable for seeder tests

Summary:
As requested in discussion in D4276. This allows us to run seeder unit tests
independently from the other unit tests run by test_bitcoin

Depends on D4432

Test Plan:

../configure
make check
src/seeder/test_bitcoin-seeder
../configure CXX=clang++ CC=clang
make clean
make check
src/seeder/test_bitcoin-seeder

ninja
ninja check

These should build everything including the seeder and seeder tests.

ninja test_bitcoin-seeder

This should only build test_bitcoin-seeder

ninja check-bitcoin-seeder

This should only run the bitcoin-seeder tests.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4417