Add IGEMM padding pointer testing

For LHS shapes which has more than one row, set the first row of data to
be padding.

As this further increases the number of different test inputs this
change also extends the caching mechanism to use an unordered map to
store the generated test data, and uses a single object which
encompasses all parameters used to generate test data

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

Approved-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>