sched_ext: Fix enq_last_no_enq_fails selftest

cc9877fb ("sched_ext: Improve error reporting during loading") changed
how load failures are reported so that more error context can be
communicated. This breaks the enq_last_no_enq_fails test as attach no longer
fails. The scheduler is guaranteed to be ejected on attach completion with
full error information. Update enq_last_no_enq_fails so that it checks that
the scheduler is ejected using ops.exit().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Vishal Chourasia <vishalc@linux.ibm.com>
Link: http://lkml.kernel.org/r/Zxknp7RAVNjmdJSc@linux.ibm.com
Fixes: cc9877fb ("sched_ext: Improve error reporting during loading")