PT-2119 - pt-osc aborts in 8.0.15 even if no FK exists

- Moved check after we identified child tables
- Added test case

t/pt-online-schema-change/samples/pt-2119.sql

@@ -0,0 +1,26 @@
+DROP DATABASE IF EXISTS pt_osc;
+CREATE DATABASE pt_osc;
+USE pt_osc;
+
+CREATE TABLE t(
+    id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
+    f1 INT
+) ENGINE=InnoDB;
+
+CREATE TABLE `person` (
+ `id` bigint(20) NOT NULL AUTO_INCREMENT,
+ `name` varchar(20) NOT NULL,
+ `testId` bigint(20) DEFAULT NULL,
+ PRIMARY KEY (`id`)
+) ENGINE=InnoDB;
+
+CREATE TABLE `test_table` (
+  `id` bigint(20) NOT NULL AUTO_INCREMENT,
+  `refId` bigint(20) DEFAULT NULL,
+  `person` bigint(20) DEFAULT NULL,
+  PRIMARY KEY (`id`),
+  KEY `fk_person` (`person`),
+  KEY `fk_refId` (`refId`),
+  CONSTRAINT `fk_person` FOREIGN KEY (`person`) REFERENCES `person`(`id`),
+  CONSTRAINT `fk_refId` FOREIGN KEY (`refId`) REFERENCES `test_table`(`id`)
+) ENGINE=InnoDB;