test-opt: fix backend support check #15317
                
     Merged
            
            
          
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The tests in
tests/test-optwere never activated in the llama.cpp repository until #13873 . The problem is thatggml_optinternally tries to allocate AdamW/SGD optimizer steps which are not supported in all backends. So it becomes necessary to check support for these optimizer steps ahead of time (otherwise the intentionally simple functionggml_opt_fitwould need to be made overly complicated). This PR allocates simple test tensors for the ops in the test code, if the ops are not supported the tests are skipped.