for #601: add toString for display test case name

sharding-jdbc-core/src/test/java/io/shardingjdbc/dbtest/config/bean/AssertDDLDefinition.java

@@ -63,4 +63,9 @@ public class AssertDDLDefinition implements AssertDefinition {
     
     @XmlElement(name = "subAssert")
     private List<AssertSubDefinition> subAsserts = new ArrayList<>();
+    
+    @Override
+    public String toString() {
+        return id;
+    }
 }

sharding-jdbc-core/src/test/java/io/shardingjdbc/dbtest/config/bean/AssertDMLDefinition.java

@@ -63,4 +63,9 @@ public class AssertDMLDefinition implements AssertDefinition {
     
     @XmlElement(name = "subAssert")
     private List<AssertSubDefinition> subAsserts = new ArrayList<>();
+    
+    @Override
+    public String toString() {
+        return id;
+    }
 }

sharding-jdbc-core/src/test/java/io/shardingjdbc/dbtest/config/bean/AssertDQLDefinition.java

@@ -54,4 +54,9 @@ public class AssertDQLDefinition implements AssertDefinition {
     
     @XmlElement(name = "subAssert")
     private List<AssertSubDefinition> subAsserts = new ArrayList<>();
+    
+    @Override
+    public String toString() {
+        return id;
+    }
 }