tests/kernel/common: Extend nested_irq_offload case to do a context switch

Bug #45779 discovered an edge case with nested interrupts on Xtensa
where they might select an incorrect thread context to return to
instead of the (mandatory!) return to the outer interrupt context.

Cleverly adjust the nested_irq_offload to exercise this.  It now
creates a thread that it knows it will interrupt, then suspends that
thread from within the inner/nested interrupt.  This guarantees that
_current will be different on exit from the second interrupt, which is
the case that tripped up Xtensa.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>