[Kernels][GPU]: fix memset on test_matmul()

The test infrastructure has a critical bug where the output buffer is never
zeroed before kernel execution. In run_test(), line 132 incorrectly zeros
c_device_buffer_ref (the reference buffer) instead of c_device_buffer (the
output buffer that kernels write to).

The matmul kernels use an accumulation pattern:
    var dst_reg: c.element_type = 0
    dst_reg += a * b  // accumulate in register
    dst[row, col] += dst_reg  // accumulate into output buffer

This requires the destination buffer to be zero-initialized. Without zeroing,
the kernel accumulates into garbage or previous test results.

Since the test instance reuses the same output buffer across multiple kernel
tests (k1 through k6), each test accumulates on top of previous results:
    - First test: c_device_buffer = garbage + result1 → WRONG
    - Second test: c_device_buffer = (garbage + result1) + result2 → MORE WRONG
    - Subsequent tests continue accumulating corrupted results

The bug existed from the original commit but likely went unnoticed because:
1. Modern GPU drivers (CUDA/ROCm) often zero-initialize allocated memory as a
   security feature to prevent data leakage between processes
2. GPU memory allocators may return pre-zeroed blocks from memory pools
3. Platform-specific behavior (NVIDIA vs AMD) may mask the issue differently
4. Test tolerance (rtol=0.01) may hide small accumulation errors

This is undefined behavior that happens to "work" on some platforms but can
fail randomly depending on driver behavior, hardware, memory allocation
patterns, or test execution order.

The fix changes line 121 to correctly zero the output buffer before each test:
    ctx.enqueue_memset(self.c_device_buffer, 0)

This ensures:
- Output buffer starts at zero before each kernel execution
- Accumulation pattern works correctly: 0 + result = result
- Tests are isolated and don't interfere with each other
- Results are deterministic and reproducible across platforms

Fixes: 830ce10 ("[Kernels] Open source the gpu tests (#60860)")

Author: mcgrof

max/kernels/test/gpu/layout/test_matmul.mojo

@@ -113,7 +113,7 @@ struct test_matmul[
         print("=== test_matmul")
 
         var ctx = self.ctx
-        ctx.enqueue_memset(self.c_device_buffer_ref, 0)
+        ctx.enqueue_memset(self.c_device_buffer, 0)
 
         fn create_tensor[
             layout: Layout