Add negative testing for avg_pool2d, max_pool2d

 * Negative tests for ERROR_IFs given in spec
 * Constrict dimension size of latter ranks if
rank is larger than 4

Signed-off-by: Matthew Haddon <matthew.haddon@arm.com>
Change-Id: Iffea1874e876dba83c8a7c63049283bf7b3ba74b