Add unit tests for CSeederNode::ProcessMessage()

Summary:
Adds some unit tests for the CSeederNode class by using a test class wrapper
for CSeederNode.

Depends on D4449

Test Plan:

../configure
make check
cmake -GNinja ..
ninja check-bitcoin-seeder

../configure CXX=clang++ CC=clang --with-gui=no
make check
cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -GNinja ..
ninja check-bitcoin-seeder

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

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

Differential Revision: https://reviews.bitcoinabc.org/D4436