[CMAKE] Fix the check-bitcoin-* targets when running with Xcode

Summary:
This diff makes the unit test runner script use the full path to run
test_bitcoin, making it independent of the generator configuration for
Xcode and Visual Studio (see D5338). This applies as well to
test_bitcoin-qt and test_bitcoin-seeder.

Note: the check target still fails and will require more changes to
make it work. This is because the python scripts bitcoin-util-test.py
use an hardcoded path to the bitcoin-tx binary (relative to the build
directory).

Test Plan:

cmake -GNinja ..
ninja check

cmake -G"Unix Makefiles" ..
make check

On OSX:

cmake -GXcode ..
cmake --build . --config RelWithDebInfo --target check-bitcoin
cmake --build . --config RelWithDebInfo --target check-bitcoin-qt
cmake --build . --config RelWithDebInfo --target check-bitcoin-seeder

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5342