Migrate boxInline tests to new multi-file framework

compiler/testData/codegen/boxInline/complexStack/simpleExtension.kt

@@ -0,0 +1,15 @@
+// FILE: 1.kt
+
+package test
+
+inline fun foo(x: String) = x
+
+inline fun processRecords(s: String?, block: String.(String) -> String) = s?.block(foo("O"))
+
+// FILE: 2.kt
+
+import test.*
+
+fun box(): String? {
+    return processRecords("O") { this + "K" }
+}