Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/samWrappersDifferentFiles.kt

@@ -0,0 +1,22 @@
+// FILE: 1/wrapped.kt
+
+fun getWrapped1(): Runnable {
+    val f = { }
+    return Runnable(f)
+}
+
+// FILE: 2/wrapped2.kt
+
+fun getWrapped2(): Runnable {
+    val f = { }
+    return Runnable(f)
+}
+
+// FILE: box.kt
+
+fun box(): String {
+    val class1 = getWrapped1().javaClass
+    val class2 = getWrapped2().javaClass
+
+    return if (class1 != class2) "OK" else "Same class: $class1"
+}