Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/mainInFiles.kt

@@ -0,0 +1,40 @@
+// FILE: 1.kt
+
+package test
+
+class A {}
+
+fun getMain(className: String): java.lang.reflect.Method {
+    val classLoader = A().javaClass.classLoader
+    return classLoader.loadClass(className).getDeclaredMethod("main", Array<String>::class.java)
+}
+
+fun box(): String {
+    val bMain = getMain("pkg.AKt")
+    val cMain = getMain("pkg.BKt")
+
+    val args = Array(1, { "" })
+
+    bMain.invoke(null, args)
+    cMain.invoke(null, args)
+
+    return args[0]
+}
+
+
+
+// FILE: a.kt
+
+package pkg
+
+fun main(args: Array<String>) {
+    args[0] += "O"
+}
+
+// FILE: b.kt
+
+package pkg
+
+fun main(args: Array<String>) {
+    args[0] += "K"
+}