Fix linalg.trace test, result dtype was incorrect

As the spec says, the output dtype should be the default
integer/float/complex dtype just like for `sum`. Given that the
reference was implemented with `sum`, avoiding to pass an explicit
dtype should be enough to obtain the correct results.

Author: rgommers

Diff for: array_api_tests/test_linalg.py

@@ -626,7 +626,7 @@ def true_trace(x_stack):
             x_stack_diag = [x_stack[i, i + offset] for i in range(diag_size)]
         else:
             x_stack_diag = [x_stack[i - offset, i] for i in range(diag_size)]
-        return _array_module.sum(asarray(x_stack_diag, dtype=x.dtype), dtype=x.dtype)
+        return _array_module.sum(asarray(x_stack_diag, dtype=x.dtype))
 
     _test_stacks(linalg.trace, x, **kw, res=res, dims=0, true_val=true_trace)