[pytest] Don't return values from test_* functions (#14475)

* [pytest] Don't return values from test_* functions

Pytest expects test functions to return None, and errors to be flagged
via exceptions. It actually emits warnings for "return" statements
encountered in test_ functions.

* Add explicit asserts

* Use not-None in assertion

tests/python/contrib/test_hexagon/test_2d_physical_buffers.py

@@ -331,7 +331,7 @@ def test_cache_shape(self, ir_module, input_layout, working_layout, output_layou
             assert len(buffer.shape) == expected_physical_dimensions
 
     def test_lower(self, schedule_args):
-        return tvm.lower(*schedule_args)
+        assert tvm.lower(*schedule_args)
 
     @requires_hexagon_toolchain
     def test_build(self, schedule_args, target_host, input_layout, working_layout, output_layout):

tests/python/contrib/test_hexagon/test_maxpool2d_blocked.py

@@ -151,7 +151,7 @@ def test_maxpool(self, shape_nhwc, window_size, stride, pad, dtype, target):
             padding=(pad, pad, pad, pad),
             dtype=dtype,
         )
-        return output, ref_output
+        assert all([output is not None, ref_output is not None])
 
 
 if __name__ == "__main__":