test: event: Correct usage of IS_ENABLED() macro in test/common/event.c

This file was using IS_ENABLED() to test for CONFIG flags but omitted
the CONFIG_ prefix and so did not work as expected.

Signed-off-by: Tom Rini <trini@konsulko.com>

test/common/event.c

@@ -91,7 +91,7 @@ static int test_event_probe(struct unit_test_state *uts)
 	struct test_state state;
 	struct udevice *dev;
 
-	if (!IS_ENABLED(SANDBOX))
+	if (!IS_ENABLED(CONFIG_SANDBOX))
 		return -EAGAIN;
 
 	state.val = 0;