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/expected/application_name_unique.out

@@ -0,0 +1,38 @@
+Create EXTENSION pg_stat_monitor;
+SELECT pg_stat_monitor_reset();
+ pg_stat_monitor_reset 
+-----------------------
+ 
+(1 row)
+
+Set application_name = 'naeem' ; 
+SELECT 1 AS num;
+ num 
+-----
+   1
+(1 row)
+
+Set application_name = 'psql' ; 
+SELECT 1 AS num;
+ num 
+-----
+   1
+(1 row)
+
+SELECT query,application_name FROM pg_stat_monitor ORDER BY query, application_name COLLATE "C";
+             query              |          application_name          
+--------------------------------+------------------------------------
+ SELECT $1 AS num               | naeem
+ SELECT $1 AS num               | psql
+ SELECT pg_stat_monitor_reset() | pg_regress/application_name_unique
+ Set application_name = 'naeem' | naeem
+ Set application_name = 'psql'  | psql
+(5 rows)
+
+SELECT pg_stat_monitor_reset();
+ pg_stat_monitor_reset 
+-----------------------
+ 
+(1 row)
+
+DROP EXTENSION pg_stat_monitor;