Migrate boxInline tests to new multi-file framework

compiler/testData/codegen/boxInline/private/nestedInPrivateClass.kt

@@ -0,0 +1,30 @@
+// FILE: 1.kt
+
+package test
+
+private class S public constructor() {
+    class Z {
+        fun a(): String {
+            return "K"
+        }
+    }
+}
+// This function exposes S.Z which is a class nested into a private class S (package-private in the byte code)
+// It can be accessed outside the `test` package now that S.Z. is public in the byte code, but it may be changed later
+internal inline fun call(s: () -> String): String {
+    return s() + test().a()
+}
+
+private fun test(): S.Z {
+    return S.Z()
+}
+
+// FILE: 2.kt
+
+import test.*
+
+fun box(): String {
+    return call {
+        "O"
+    }
+}