Create test/test_custom_ops.py, move test_custom_op_testing to it (#106035)
I'm in the process of putting all the custom op tests into this file. I
got tired of trying to find the custom ops tests in
test_python_dispatch.py, which (1) is getting long and (2) should actually
be the torch_dispatch and python torch.library tests.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106035
Approved by: https://github.com/bdhirsh, https://github.com/soulitzer
