[mlir][sparse] add an elaborate sparse storage scheme integration test

Looks "under the hood" of the sparse stogage schemes.
Users should typically not be interested in these details
(hey, that is why we have "sparse compilers"!) but this
test makes sure the compact contents are as expected.

Reviewed By: ThomasRaoux, bixia

Differential Revision: https://reviews.llvm.org/D107683