[IR] Reorganize test files in involvesIrInterpreter dir

compiler/testData/codegen/box/involvesIrInterpreter/stringConcatenation.kt

@@ -0,0 +1,16 @@
+// TARGET_BACKEND: JVM_IR
+// TARGET_BACKEND: NATIVE
+// TARGET_BACKEND: JS_IR
+// WITH_STDLIB
+
+const val simple = <!EVALUATED("OK 3.5")!>"O${'K'} ${1.toLong() + 2.5}"<!>
+const val withInnerConcatenation = <!EVALUATED("1 2 3 4 5 6")!>"1 ${"2 ${3} ${4} 5"} 6"<!>
+const val withNull = <!EVALUATED("1 null")!>"1 ${null}"<!> // but `"1" + null` is invalid
+
+fun box(): String {
+    if (<!EVALUATED("false")!>simple != "OK 3.5"<!>) return "Fail 1"
+    if (<!EVALUATED("false")!>withInnerConcatenation != "1 2 3 4 5 6"<!>) return "Fail 2"
+    if (<!EVALUATED("false")!>withNull != "1 null"<!>) return "Fail 3"
+
+    return "OK"
+}