Refactor kwargs validation tests based on feedback

Address review feedback from OriolAbril on PR arviz-devs#314:

- Only test accessor methods that accept **kwargs
- Use pytest.mark.parametrize to reduce test duplication
- Remove test docstrings (test names are self-explanatory)
- Use importorskip pattern for minimal environment support
- Use datatree fixture from conftest.py instead of redefining
- Remove INVESTIGATION_SUMMARY.md from repository
- Import numpy at module level instead of inside test functions
- Follow the same pattern as tests in PR arviz-devs#143

The parametrized test covers 20 accessor methods, with 3 additional
tests for methods that require positional arguments before **kwargs.

All 23 tests pass successfully.

Author: Shlokpalrecha