PT-153 Added --[no]fail-on-unique-key-change param

t/pt-online-schema-change/samples/pt-153.sql

@@ -0,0 +1,19 @@
+DROP DATABASE IF EXISTS test;
+CREATE DATABASE test;
+USE test;
+CREATE TABLE test.t1 (
+id int,
+f2 int, 
+f3 int NULL,
+f4 int,
+PRIMARY KEY (id)
+);
+
+INSERT INTO test.t1 VALUES
+(1,1,1,1), 
+(2,1,1,1), 
+(3,1,2,1), 
+(4,2,NULL,2), 
+(5,3,NULL,2),
+(6,4,4,4), 
+(7,4,4,4);