[AUTOTOOLS] Don't build the seeder tests if --disable-tests is set

Summary:
The seeder tests are conditioned to the seeder build, but it must be
built only if the seeder is built *AND* the tests are enabled.
If the tests are not enabled there is no check for the boost unit tests
library.

Test Plan:

./autogen.sh
mkdir build && cd build
../configure
make

Check that bitcoin-seeder and test_bitcoin-seeder are built.

../configure --disable-tests
make

Check that bitcoin-seeder is built but not test_bitcoin-seeder.

../configure --without-seeder
make

Check that neither bitcoin-seeder nor test_bitcoin-seeder is built.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, nakihito

Reviewed By: #bitcoin_abc, jasonbcox, nakihito

Differential Revision: https://reviews.bitcoinabc.org/D4767