Skip TestNNAPI tests if QNNPACK is not supported (#82882)
### Description
As the testcase unconditionally uses qnnpack skip the test if that isn't supported like other similar tests.
Fixes https://github.com/pytorch/pytorch/issues/82810
### Testing
Ran `test_nnapi.py` on PPC
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82882
Approved by: https://github.com/albanD
