Added new string tests

compiler/testData/codegen/box/strings/interpolation.kt

@@ -0,0 +1,9 @@
+fun test(p: String?): String {
+    return "${p ?: "Default"} test"
+}
+fun box(): String {
+    if (test(null) != "Default test") return "fail 1: ${test(null)}"
+    if (test("Good") != "Good test") return "fail 1: ${test("OK")}"
+
+    return "OK"
+}