PG-267 Add testcase to test histogram.

This commit adds following three sql based testcases:

1) Test unique application name set by user.
2) Histogram function is working properly as desired.
3) Error on insert is shown with proper message.

regression/sql/error_insert.sql

@@ -0,0 +1,17 @@
+Drop Table if exists Company;
+
+CREATE TABLE Company(
+   ID INT PRIMARY KEY     NOT NULL,
+   NAME TEXT    NOT NULL
+);
+
+
+CREATE EXTENSION pg_stat_monitor;
+SELECT pg_stat_monitor_reset();
+INSERT  INTO Company(ID, Name) VALUES (1, 'Percona'); 
+INSERT  INTO Company(ID, Name) VALUES (1, 'Percona'); 
+
+Drop Table if exists Company;
+SELECT query, elevel, sqlcode, message FROM pg_stat_monitor ORDER BY query COLLATE "C",elevel;
+SELECT pg_stat_monitor_reset();
+DROP EXTENSION pg_stat_monitor;