Fill in missing atomic tests and unify

[rhornung67]

Is your feature request related to a problem? Please describe.

In the test/unit/atomic directory, we have uneven testing of atomics across CUDA, HIP, and OpenMP, and no atomic testing for SYCL or OpenMP target offload.

Describe the solution you'd like

Work through the atomic test we have to make sure all atomic operations are tested, including RAJA atomics and desul atomics, for all RAJA back-ends. Try to unify as much as possible to eliminate conditionally-compiled code where possible and follow the test parameterization patterns we use in other tests.

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Additional context

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