Add new sparse matmul performance tracking test

Existing test for sparse matrix-matrix multiplication ("sparse" matrix of all ones) is not very representative.

Test matrix is thanks to Michael Schnall-Levin here https://groups.google.com/forum/#!topic/julia-users/TVbVXtbNzgk

Does anything else need to be added, or would this automatically start showing up on speed.julialang.org if it gets merged?