Add regression test for pg_backend_memory_contexts.

Author: Atsushi Torikoshi
Reviewed-by: Michael Paquier, Fujii Masao
Discussion: https://postgr.es/m/20200819135545.GC19121@paquier.xyz

src/test/regress/expected/sysviews.out

@@ -19,6 +19,15 @@ select count(*) >= 0 as ok from pg_available_extensions;
  t
 (1 row)
 
+-- The entire output of pg_backend_memory_contexts is not stable,
+-- we test only the existance and basic condition of TopMemoryContext.
+select name, ident, parent, level, total_bytes >= free_bytes
+  from pg_backend_memory_contexts where level = 0;
+       name       | ident | parent | level | ?column? 
+------------------+-------+--------+-------+----------
+ TopMemoryContext |       |        |     0 | t
+(1 row)
+
 -- At introduction, pg_config had 23 entries; it may grow
 select count(*) > 20 as ok from pg_config;
  ok 

src/test/regress/sql/sysviews.sql

@@ -12,6 +12,11 @@ select count(*) >= 0 as ok from pg_available_extension_versions;
 
 select count(*) >= 0 as ok from pg_available_extensions;
 
+-- The entire output of pg_backend_memory_contexts is not stable,
+-- we test only the existance and basic condition of TopMemoryContext.
+select name, ident, parent, level, total_bytes >= free_bytes
+  from pg_backend_memory_contexts where level = 0;
+
 -- At introduction, pg_config had 23 entries; it may grow
 select count(*) > 20 as ok from pg_config;