[mlir][sparse] add sparse kernels test to sparse compiler test suite

This test makes sure kernels map to efficient sparse code, i.e. all
compressed for-loops, no co-iterating while loops.  In addition, this
revision removes the special constant folding inside the sparse
compiler in favor of Mahesh' new generic linalg folding. Thanks!

NOTE: relies on Mahesh fix, which needs to be rebased first

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D110001