Change kernel lists to use lazy initialization

There is an issue where the order of static initializations has no
guaranteed order, which can cause test listing to be initialized before
list of kernels. This can be solved by lazily initialize kernel lists on
first use.

This patch applies this fix for `matmul_test.cpp`

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

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