Add unit test for distributed tables

This commit aims to add a unit test for executing queries on
distributed tables. Test aims to get the error message when
there are no shards created for the distributed tables.

expected/queries.out

@@ -13,6 +13,11 @@ SELECT master_create_distributed_table('articles', 'author_id');
 
 (1 row)
 
+-- test when a table is distributed but no shards created yet
+SELECT count(*) from articles;
+ERROR:  cannot plan SELECT query
+DETAIL:  There are no existing shards for the distributed table.
+HINT:  Run "master_create_worker_shards" to create shards for the distributed table.
 \set VERBOSITY terse
 SELECT master_create_worker_shards('articles', 2, 1);
 WARNING:  Connection failed to adeadhost:5432

sql/queries.sql

@@ -11,6 +11,9 @@ CREATE TABLE articles (
 
 SELECT master_create_distributed_table('articles', 'author_id');
 
+-- test when a table is distributed but no shards created yet
+SELECT count(*) from articles;
+
 \set VERBOSITY terse
 SELECT master_create_worker_shards('articles', 2, 1);
 \set VERBOSITY default