event: Add a simple test

Add a test for event registration and activation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index b534ad6..2786ada 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -814,6 +814,7 @@
 S:	Maintained
 F:	common/event.c
 F:	include/event.h
+F:	test/common/event.c
 
 FASTBOOT
 S:	Orphaned