[IR] add testdata for dumpKotlinLike

compiler/testData/ir/irText/expressions/stringPlus.kt.txt

@@ -0,0 +1,12 @@
+fun test1(a: String, b: Any): String {
+  return a.plus(other = b)
+}
+
+fun test2(a: String, b: Int): String {
+  return a.plus(other = "+").plus(other = b)
+}
+
+fun test3(a: String, b: Int): String {
+  return a.plus(other = "+").plus(other = b.plus(other = 1)).plus(other = a)
+}
+