test_irq_offload: unit test for running functions in IRQ context

This is used in many other test cases. However when implementing this
function it's helpful to have a testcase dedicated for it, without
dependencies on other kernel objects.

Change-Id: I66a7cdd0b13712665384d5ad4e79050c82d32e3a
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>