Align test terminology with kernel terminology

The test code mostly uses the description `nt_nt` to indicate that LHS
and RHS are non-transposed. This doesn't match the naming scheme used by
the kernels, which is `kxn` and `nxk`. This change aligns testing code
with the kernels in this naming regard.

Signed-off-by: Emil Ohlsson <emil.ohlsson@arm.com>