Migrate boxInline tests to new multi-file framework

compiler/testData/codegen/boxInline/special/plusAssign.kt

@@ -0,0 +1,26 @@
+// FILE: 1.kt
+
+package test
+
+public class Z(public var s: Int)
+
+operator inline fun Z.plusAssign(lambda: () -> Int)  {
+    this.s += lambda()
+}
+
+// FILE: 2.kt
+
+import test.*
+
+fun test1(s: Int): Int {
+    val z = Z(s)
+    z += {s}
+    return z.s
+}
+
+fun box(): String {
+    val result = test1(11)
+    if (result != 22) return "fail1: ${result}"
+
+    return "OK"
+}