testprogs:subunit.sh: Add function for expected failures.

The testit_expect_failure() function is like the testit() function, with
reversed error detection logic.  This reversal only affects the pass/fail logic
and logging - the original return code from the command is still returned to the
calling script.

testprogs/blackbox/subunit.sh

@@ -65,3 +65,18 @@ testit () {
 	fi
 	return $status
 }
+
+testit_expect_failure () {
+	name="$1"
+	shift
+	cmdline="$*"
+	subunit_start_test "$name"
+	output=`$cmdline 2>&1`
+	status=$?
+	if [ x$status = x0 ]; then
+		echo "$output" | subunit_fail_test "$name"
+	else
+		subunit_pass_test "$name"
+	fi
+	return $status
+}