Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/multifileClassPartsInitialization.kt

@@ -0,0 +1,26 @@
+// FILE: box.kt
+
+import a.*
+
+fun box(): String = OK
+
+// FILE: part1.kt
+
+@file:[JvmName("MultifileClass") JvmMultifileClass]
+package a
+
+val O: String = "O"
+
+// FILE: part2.kt
+
+@file:[JvmName("MultifileClass") JvmMultifileClass]
+package a
+
+val K: String = "K"
+
+// FILE: part3.kt
+
+@file:[JvmName("MultifileClass") JvmMultifileClass]
+package a
+
+val OK: String = O + K