tests: arm: irq_vector_table: Fix Kconfig override

Since on ARM CONFIG_NUM_IRQS option has no prompt, it cannot be properly
overridden by a prj.conf fragment. Instead make it have a prompt for
this particular test to be able to override it properly.

Fixes #8200

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>