tests: arch: arm_irq_vector_table: no explicit const array size

We do not need to explicitly define the size of the const
array elements for the customized IRQ vector, used in the
test. The compiler should be able to infer the actual size
by counting the provided entries.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>