Fix: always sort Python model dfs in unit tests

[georgesittas]

We sort the expected & actual dataframes for SQL model unit tests, when the ORDER BY clause is missing. This PR makes it so that we always sort them for Python models too, since we don't have a clause like that.

The added test fails when the flag is set to False:

tests/core/test_test.py:95: AssertionError
------------------------------------------------ Captured stdout call ------------------------------------------------
AssertionError: Data mismatch (exp: expected, act: actual)

   id     value
  exp act   exp act
0   1   3     a   c
1   2   1     b   a
2   3   2     c   b