Migrate boxMultiFile and boxMultifileClass tests to new multi-file tests

AbstractCompileKotlinAgainstMultifileKotlinTest is broken in this commit; will
be fixed later

compiler/testData/codegen/boxMultiFile/kt9958Interface.kt

@@ -0,0 +1,32 @@
+// FILE: a.kt
+
+package a
+
+import b.*
+
+interface B {
+    companion object : A() {}
+
+    fun test() {
+        foo()
+    }
+}
+
+class C : B
+
+fun box(): String {
+    C().test()
+    return result
+}
+
+// FILE: b.kt
+
+package b
+
+var result = "fail"
+
+abstract class A {
+    protected fun foo() {
+        result = "OK"
+    }
+}