Added new string tests

compiler/testData/codegen/bytecodeText/stringOperations/infixPlus.kt

@@ -0,0 +1,17 @@
+class A() {
+
+    override fun toString(): String {
+        return "A"
+    }
+}
+
+
+fun box() : String {
+
+    val s = "1" plus "2" plus 3 plus 4L plus 5.0 plus 6F plus '7' plus A()
+
+    return "OK"
+}
+
+/*TODO:
+1 NEW java/lang/StringBuilder*/