[TOPI][Testing] Enable conv2d NHWC fp16 topi testing for `arm_cpu`

[Anndrey24]

This commit adds fp16 test cases to the conv2d NHWC TOPI schedules for arm_cpu. Following the example of #8529, the numpy reference conv2d output is computed in fp32 instead of fp16, while the absolute tolerance varies for each test case according to the size of the summed axis and the output's largest element.

cc @ekalda @lhutton1