ktest: Fix child exit code processing

The child_exit errno needs to be shifted by 8 bits to compare against the
return values for the bisect variables.

Fixes: c5dacb88 ("ktest: Allow overriding bisect test results")
Cc: stable@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>