Make GIN test using injection points repeatable

As written, the test would fail when run repeatedly because one of the
injection points attached was not detached.  This would not matter if
the test is rewritten to be concurrently safe, but let's be clean and
it is a good practice.

Oversight in 6a1ea02c491d.

Discussion: https://postgr.es/m/ZfP7IDs9TvrKe49x@paquier.xyz

src/test/modules/gin/expected/gin_incomplete_splits.out

@@ -179,3 +179,9 @@ select verify(:next_i);
  t
 (1 row)
 
+SELECT injection_points_detach('gin-finish-incomplete-split');
+ injection_points_detach 
+-------------------------
+ 
+(1 row)
+

src/test/modules/gin/sql/gin_incomplete_splits.sql

@@ -143,3 +143,5 @@ select insert_n(:next_i, 10) as next_i
 \gset
 -- Verify that a scan still works
 select verify(:next_i);
+
+SELECT injection_points_detach('gin-finish-incomplete-split');