[mlir][sparse] make test for block sparsity more robust (#74798)

For BSR and convolutions, we encounter

(d0, d1, d2, d3) -> ((d0 + d2) floordiv 2, (d1 + d3) floordiv 2, (d0 +
d2) mod 2, (d1 + d3) mod 2)

which crashed the current test. Note that an actual test and working
code is still to follow (since we need to fix a few other things first)