Reduce size of data files used in gold master tests for `unfilter_design`

[oskooi]

The gold master test for the function unfilter_design of the adjoint-solver module added as part of #2462 involved adding two data files (mpa_unfilter_design.npy and mpa_unfilter_design_target.npy) to the repository which are each 1.3MB in size. The size of these files can likely be reduced considerably without affecting the tests. (Unfortunately, I did not realize how large these files were during the review.)

cc @mochen4

[mochen4]

Thanks! I reduced the size of those files in PR #2543