dtc: Make dtc-checfails.sh script catch deaths-by-signal

Since commit 5ba0086bfd, the
dtc-checkfails.sh script does not check the return code from dtc.
That's reasonable, since depending on the checks we're testing, dtc
could either complete succesfully or return an error.

However, it's never right for dtc to SEGV or otherwise be killed by a
signal.  So the script should catch that, and fail the testcase if it
happens.  This patch implements this behaviour.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/dtc-checkfails.sh

@@ -17,6 +17,10 @@ rm -f $TMPFILE $LOG
 verbose_run_log "$LOG" "$DTC" -o /dev/null "$@"
 ret="$?"
 
+if [ "$ret" -gt 127 ]; then
+    FAIL "dtc killed by signal (ret=$ret)"
+fi
+
 for c in $CHECKS; do
     if ! grep -E "^(ERROR)|(Warning) \($c\):" $LOG > /dev/null; then
 	FAIL "Failed to trigger check \"%c\""