[CMAKE] Actually run the seeder tests

Summary:
The seeder test executable is built, but no test is declared so they are
not run. This diff fixes this bug.

Test Plan:

ninja check-bitcoin-seeder

Edit src/seeder/test/seeder_tests.cpp to make the test fail, then
re-run:

ninja check-bitcoin-seeder

Check the test failure is reported correctly.

Reviewers: #bitcoin_abc, deadalnix, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D4892