tests: Fix IRQ locking in sched benchmark

Corrects an issue that was introduced when the interrupt
locking/unlocking was added to the 'sched' benchmark by
unlocking the interrupts before the context switch done by
k_yield(), but after the call to z_unpend_first_thread().

Fixes PR #81050

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>