fixup! DRA scheduler: adapt to v1alpha3 API

Updates allocator_test.go:

- reduce amount of parameters required by helper functions
- use local functions to create objects that are only
  different in one test case (makes the difference in that
  test case more obvious, allows simplifying helpers)
- add some comments explaining why test cases are the
  way they are
- remove DeviceClass.SuitableNodes test cases, that field
  doesn't matter for structured parameters
- add more cases for network attached devices, in particular
  around the resulting node selector

Author: pohly