test: split unit tests from integration tests

[redeboer]

All tests that require AmpForm have been seperated from proper unit tests (that require no additional dependencies). The folder structure under tests is now:

tests
├── integration
└── unit

Additional improvements:

• Fixtures for the AmpForm tests have been parametrized:
  • qrules.ReactionInfo is parametrized with the canonica-helicity and helicity formalism.
  • SympyModel is constructed with and without max_complexity argument in the constructor, so that optimized_lambdify is tested as well.
• Improved error message of LambdifiedFunction.update_parameters: over-defined parameters were computed incorrectly. In addition, the error message now prints the expected parameters.
• Callbacks can now take pathlib.Path (previously only str). This makes it possible to convert the output_dir fixture into a Path as well.