Adds missing dslx_test rule for the matrix multiplication example.
Adds missing dslx_test rule for the matrix multiplication example.

Because this was missing, I missed one thing I still needed to do for full arrays-of-channels support: lowering to IR!

PiperOrigin-RevId: 450780140

google/xls,google/xls,google/xls,google/xls,google/xls,google/xls