Migrate boxInline tests to new multi-file framework

compiler/testData/codegen/boxInline/simple/vararg.kt

@@ -0,0 +1,22 @@
+// FILE: 1.kt
+
+package test
+
+inline fun doSmth(vararg a: String) : String {
+    return a.foldRight("", { a, b -> a + b})
+}
+
+// FILE: 2.kt
+
+import test.*
+
+fun test1(): String {
+    return doSmth("O", "K")
+}
+
+fun box(): String {
+    val result = test1()
+    if (result != "OK") return "fail1: ${result}"
+
+    return "OK"
+}