Fix 'implicit fallthrough' warning with new GCC

GCC 7 now warns on case statements falling through without an explicit
comment that falling through is OK. Insert some to make it happy.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>
diff --git a/tests/subsurface-test.c b/tests/subsurface-test.c
index edb0842..1a24ee3 100644
--- a/tests/subsurface-test.c
+++ b/tests/subsurface-test.c
@@ -579,22 +579,31 @@
 
 	case 11:
 		SUB_LINK(10, 2);
+		/* fallthrough */
 	case 10:
 		SUB_LINK(9, 2);
+		/* fallthrough */
 	case 9:
 		SUB_LINK(8, 6);
+		/* fallthrough */
 	case 8:
 		SUB_LINK(7, 6);
+		/* fallthrough */
 	case 7:
 		SUB_LINK(6, 2);
+		/* fallthrough */
 	case 6:
 		SUB_LINK(5, 1);
+		/* fallthrough */
 	case 5:
 		SUB_LINK(4, 3);
+		/* fallthrough */
 	case 4:
 		SUB_LINK(3, 1);
+		/* fallthrough */
 	case 3:
 		SUB_LINK(2, 0);
+		/* fallthrough */
 	case 2:
 		SUB_LINK(1, 0);