[OpenMP] HSA_ENABLE_SDMA visible in libomptarget tests (#80860)

Enable the environment variable inside the test environment. This allows
to disable SDMA engine transfers as a potential mitigation of flaky
OpenMP offloading tests on AMDGPU.

Motivated by the open ticket https://github.com/ROCm/ROCm/issues/2616
about a missed synchronization signal.