Duckarray tests for constructors and properties

[TomNicholas]

Builds on top of #4972 to add tests for Variable/DataArray/Dataset constructors and properties when wrapping duck arrays.

Adds a file xarray/tests/duckarrays/base/constructors.py which contains new test base classes.

Also uses those new base classes to test Sparse array integration (not yet tried for pint integration).

• [x] Closes part of #6894
• [ ] Tests added (tests for tests?? Maybe...)
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst