Test suite cleanup

When we migrated from unittest to pytest, our understanding was that we had to use only test functions rather than test cases with test methods. This lead to structures like this:

https://github.com/pystiche/pystiche/blob/65f4d787e44b1ffbf7e5b6e48298ed8c7460e5a9/tests/integration/enc/test_multi_layer_encoder.py#L50-L68

In general, there are quite a few places were the test suite looks like

def test_Foo_bar():
    ...

def test_Foo_baz():
    ...

instead of

class TestFoo:
    def test_bar(self):
        ...

    def test_baz(self):
        ...

Cleaning this up is a little bit of busy work, but can be a good entry point to the development of pystiche.

[DevPranjal]

Tracker for tests in tests/integration/:

• core (#520)
• data (#520)
• demo (#523)
• enc (#537)
  • models (#523)
• image (#536)
• loss (#536)
• meta (#537)
• misc (#537)
• ops (#539)
• pyramid (#539)

[DevPranjal]

Looking forward to help more…