dtrace tests: Diff expected and actual output upon a miscompare

This makes quick diagnosis of test failures easier.

MFC after:	1 week

(cherry picked from commit a61d2c7fbd)

cddl/usr.sbin/dtrace/tests/tools/dtest.sh

@@ -119,6 +119,10 @@ test stdout:
 --
 $(cat $STDOUT)
 --
+test stdout diff:
+--
+$(diff -u $EXOUT $STDOUT)
+--
 __EOF__
     fi
     if [ $(stat -f '%z' $STDERR) -gt 0 ]; then