`TestDeleteIgnoreWithFK` is flaky

[YangKeao]

Enhancement

The test TestDeleteIgnoreWithFK is flaky, because the warnings may have different orders.

    result.go:49: 
        	Error Trace:	pkg/testkit/result.go:49
        	            				pkg/executor/delete_test.go:142
        	Error:      	Not equal: 
        	            	expected: "[Warning 1451 Cannot delete or update a parent row: a foreign key constraint fails (`test`.`child`, CONSTRAINT `fk_1` FOREIGN KEY (`a`) REFERENCES `parent` (`a`))]\n[Warning 1451 Cannot delete or update a parent row: a foreign key constraint fails (`test`.`child2`, CONSTRAINT `fk_1` FOREIGN KEY (`a`) REFERENCES `parent2` (`a`))]\n"
        	            	actual  : "[Warning 1451 Cannot delete or update a parent row: a foreign key constraint fails (`test`.`child2`, CONSTRAINT `fk_1` FOREIGN KEY (`a`) REFERENCES `parent2` (`a`))]\n[Warning 1451 Cannot delete or update a parent row: a foreign key constraint fails (`test`.`child`, CONSTRAINT `fk_1` FOREIGN KEY (`a`) REFERENCES `parent` (`a`))]\n"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1,3 +1,3 @@
        	            	+[Warning 1451 Cannot delete or update a parent row: a foreign key constraint fails (`test`.`child2`, CONSTRAINT `fk_1` FOREIGN KEY (`a`) REFERENCES `parent2` (`a`))]
        	            	 [Warning 1451 Cannot delete or update a parent row: a foreign key constraint fails (`test`.`child`, CONSTRAINT `fk_1` FOREIGN KEY (`a`) REFERENCES `parent` (`a`))]
        	            	-[Warning 1451 Cannot delete or update a parent row: a foreign key constraint fails (`test`.`child2`, CONSTRAINT `fk_1` FOREIGN KEY (`a`) REFERENCES `parent2` (`a`))]
        	            	 
        	Test:       	TestDeleteIgnoreWithFK
        	Messages:   	sql:show warnings, args:[]