[testing] Create the CMake infrastructure for cudaq/unittests

[cabreraam]

Motivation

Currently, there is a directory in unittests called Optimizer that doesn't really belong there.
The other tests in unittests are runtime-focused, while the tests in Optimizer stress components of the CUDA-Q compiler.
Thus they really belong in cudaq/test.

Goal

The overall goal is to create the infrastructure for cudaq/unittests so that we can isolate the testing of compiler components from the runtime-focused component testing in unittests. This will be in the style of how llvm/unittest and llvm/test/Unit are structured in llvm-project.

Implementation

This PR will be the first of two.
This PR defines the directory and the CMake infrastructure for cudaq/unittests.
In particular, we take inspiration from add_llvm_unittest when implementing add_cudaq_unittest.

Notes

This PR contributes toward #4684

[github-actions]

CI Summary (push) — ✅ passed

Run #27308008416 · ✅ 6 · ⏩ 7 · ❌ 0 · ⛔ 0

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Job	Result
binaries	⏩ skipped
build_and_test	✅ success
config_devdeps	✅ success
config_source_build	⏩ skipped
config_wheeldeps	✅ success
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metadata	✅ success
python_metapackages	⏩ skipped
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source_build	⏩ skipped
wheeldeps	✅ success

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Job
binaries
config_source_build
docker_image
gen_code_coverage
python_metapackages
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Build and test (amd64, gcc12, openmpi) / Dev environment (Debug)	✅ success	view
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Required check	Status	Link
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