ENH: Missing tests for relational methods with Stairs (over datetime domains)

[venaturum]

Describe the solution you'd like

We are missing relational method tests with dates (we only have test_eq and test_ne). They belong here

https://github.com/staircase-dev/staircase/blob/00da1209eec21d3ea11f0f18f538637d2bae5ecf/tests/test_dates/test_dates_relational.py

Additional context

Compare with the tests for relational methods over floats domain
https://github.com/staircase-dev/staircase/blob/00da1209eec21d3ea11f0f18f538637d2bae5ecf/tests/test_floats/test_floats_relational.py