Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/callMultifileClassMemberFromOtherPackage.kt

@@ -0,0 +1,22 @@
+// FILE: box.kt
+
+package test
+
+import b.bar
+
+fun box(): String = bar()
+
+// FILE: caller.kt
+
+package b
+
+import a.foo
+
+fun bar(): String = foo()
+
+// FILE: multifileClass.kt
+
+@file:[JvmName("MultifileClass") JvmMultifileClass]
+package a
+
+fun foo(): String = "OK"