issues: 512564022

As a follow-up to #3238, this adds tests for datasets. Replacing assert_equal_with_units with assert_identical and adding tests for the toplevel functions are not included, these will be new PRs. - [x] Tests added - [x] Passes black . && mypy . && flake8

As a reference for myself, this is the list of methods from the documentation of Dataset: - [x] creation: Dataset() - [x] contents: copy, assign, assign_coords, assign_attrs, pipe, merge, rename, rename_vars, rename_dims, swap_dims, expand_dims, drop, drop_dims, set_coords, reset_coords - [x] comparisons: equals, broadcast_equals, identical - [x] indexing: loc, isel, sel, head, tail, thin, squeeze, interp, interp_like, reindex, reindex_like, set_index, reset_index, reorder_levels - [x] missing value handling: isnull, notnull, combine_first, count, dropna, fillna, ffill, bfill, interpolate_na, where, isin - [x] computation: apply, reduce, groupby, groupby_bins, rolling, rolling_exp, coarsen, resample, diff, quantile, differentiate, integrate - [x] aggregation: all, any, argmax, argmin, max, min, mean, median, prod, sum, std, var - [x] ndarray methods: astype, argsort, clip, conj, conjugate, imag, round, real, cumsum, cumprod, rank - [x] grouped operations: assign, assign_coords, first, last, fillna, where, quantile - [x] reshaping and reorganizing: transpose, stack, unstack, to_stacked_array, shift, roll, sortby

these methods are not covered: - creation: decode_cf - dictionary interface: __getitem__, __setitem__, __delitem__, update, items, values - broadcast_like (since I also forgot this for DataArray, I'll create a new PR for it)