Add regression test to catch future breakage of avg(interval). This

aggregate seems uniquely fragile, because it's the only one with an
agginitval that's at all likely to change in format.

src/test/regress/expected/interval.out

@@ -212,3 +212,12 @@ SELECT '' AS ten, INTERVAL_TBL.*;
      | @ 5 mons 12 hours
 (10 rows)
 
+-- test avg(interval), which is somewhat fragile since people have been
+-- known to change the allowed input syntax for type interval without
+-- updating pg_aggregate.agginitval
+select avg(f1) from interval_tbl;
+                       avg                       
+-------------------------------------------------
+ @ 4 years 1 mon 10 days 4 hours 18 mins 23 secs
+(1 row)
+

src/test/regress/sql/interval.sql

@@ -60,3 +60,9 @@ SELECT '' AS fortyfive, r1.*, r2.*
 SET DATESTYLE = 'postgres';
 
 SELECT '' AS ten, INTERVAL_TBL.*;
+
+-- test avg(interval), which is somewhat fragile since people have been
+-- known to change the allowed input syntax for type interval without
+-- updating pg_aggregate.agginitval
+
+select avg(f1) from interval_tbl;