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).