testing: fix affine maps in matmul test

[superlopuh]

A minor thing I spotted recently, weirdly doesn't seem to affect things when lowering to a perfect loop nest with memref reads and writes, the loops are just permuted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.64%. Comparing base (16954e0) to head (274c523).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2666   +/-   ##
=======================================
  Coverage   89.64%   89.64%           
=======================================
  Files         361      361           
  Lines       46254    46254           
  Branches     6992     6992           
=======================================
  Hits        41464    41464           
  Misses       3719     3719           
  Partials     1071     1071           

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