tests interrupt_offload: Set a proper interrupt for POSIX arch targets

The test assumed that interrupt line 5 was up for grabs, but it
is not in general. (For ex., on an nrf53_bsim this is the clock
interrupt, which cannot be hijacked).

Instead, for boards that define it, let's use the int line
used for offloading SW interrupts (which is defined for all posix
arch boards in tree)
And if this is not defined, let's skip the test.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>