Fix two issues with recently-introduced nbtree test

REGRESS has forgotten about the test nbtree_half_dead_pages, and a
.gitignore was missing from the module.

Oversights in c085aab278 for REGRESS and 1e4e5783e7 for the missing
.gitignore.

Discussion: https://postgr.es/m/aTipJA1Y1zVSmH3H@paquier.xyz

src/test/modules/nbtree/.gitignore

@@ -0,0 +1,4 @@
+# Generated subdirectories
+/log/
+/results/
+/tmp_check/

src/test/modules/nbtree/Makefile

@@ -2,7 +2,8 @@
 
 EXTRA_INSTALL = src/test/modules/injection_points contrib/amcheck
 
-REGRESS = nbtree_incomplete_splits
+REGRESS = nbtree_half_dead_pages \
+	nbtree_incomplete_splits
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config