Removing usage of setTargetBackend from unittests

[Renaud-K]

This is the second step towards removing cudaq::set_target_backend and quantum_platform::setTargetBackend) from the public interfaces. In this step, all usages of quantum_platform::setTargetBackend) are removed from the tests.

Instead of calling

  auto &platform = cudaq::get_platform();
  platform.setTargetBackend(backendString);

the tests now link in backendConfig.cpp and set target through NVQPP_TARGET_BACKEND_CONFIG.

Tests had to be split for each configuration, since the configuration can now only be set at compile time.
Some setups had to be moved from the test into the invoking script because the information is needed before the test enters main.

There are also some clean-ups to use the default cmake settings and variables.

The last step will be to remove the calls from the public interfaces, however this will have some impact on the documentation so it will be a separate PR.

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[Renaud-K]

/ok to test 63e5073

Command Bot: Processing...

[github-actions]

CUDA Quantum Docs Bot: A preview of the documentation can be found here.

[Renaud-K]

/ok to test 3e06b53

Command Bot: Processing...