[CMAKE] Make the test python scripts depend on targets and not on files
[CMAKE] Make the test python scripts depend on targets and not on files

Summary:
The python test scripts `test_runner.py` and `bitcoin-util-test.py`
require the `bitcoind`, `bitcoin-cli` and `bitcoin-tx` binaries to run.
The dependency to these binaries is actually hardcoded as a relative
path, rather than relying on cmake target path substitution. This will
defeat the configuration aware generators that append a configuration
directory to the path.

Test Plan:
  cmake -GNinja ..
  ninja check check-functional

On OSX:
  cmake -GXcode ..
  CONFIG=RelWithDebInfo
  export BITCOINCLI=${PWD}/src/${CONFIG}/bitcoin-cli
  export BITCOIND=${PWD}/src/${CONFIG}/bitcoind
  cmake --build . --config ${CONFIG} --target check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5340

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