Roundtrip dict and json kerchunk tests without depending on kerchunk library

[jsignell]

It was discussed yesterday that a lot of the roundtrip test could run pretty much as-written if we just make a little helper that we can substitute in for xr.open_dataset(..., engine="kerchunk").

Making that substitution seems valuable because then a lot more of the test suite would run even in the absence of kerchunk.

This could be done as an alternative to or in addition to #376

[jsignell]

Closing this - since as of fsspec/kerchunk#516 the backend code is much more significant