Use BUILD_TESTS to control OpenCL tests

[jpsamaroo]

It's unintuitive that you need to set both BUILD_TESTS=OFF and BUILD_TESTING=OFF to disable building all tests. This changes the OpenCL tests to instead just check BUILD_TESTS.

[vsytch]

Thank you for bringing this issue up. We are currently testing this change internally. We are also planning to remove the bundled OpenCL ICD from the tree. I cannot give an ETA for this change, but most likely it will be publicly available with ROCm 5.0.