Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/samePartNameDifferentFacades.kt

@@ -0,0 +1,21 @@
+// FILE: 1/part.kt
+
+@file:JvmName("Foo")
+@file:JvmMultifileClass
+package test
+
+fun foo(): String = "O"
+
+// FILE: 2/part.kt
+
+@file:JvmName("Bar")
+@file:JvmMultifileClass
+package test
+
+fun bar(): String = "K"
+
+// FILE: box.kt
+
+package test
+
+fun box(): String = foo() + bar()