Use lazy initialization in unit tests.

Static initialization has no guaranteed order, which may cause test
listing to be initialized before the list of kernels.

This fixes unit tests
- matmul_clamp_f32_bf16p_bf16p_test
- matmul_clamp_f16_bf16p_bf16p_test

Signed-off-by: Jens Elofsson <jens.elofsson@arm.com>

Approved-by: Anton Bondarenko <anton.bondarenko@arm.com>