[K/N][tests] Possible intermediate library for interop test

Now along with earlier [interop] <- [main] scheme one can also use
[interop] <- [lib] <- [main] which previously was only for link tests

kotlin-native/build-tools/src/main/kotlin/org/jetbrains/kotlin/KotlinNativeTest.kt

@@ -471,6 +471,10 @@ open class KonanInteropTest : KonanStandaloneTest() {
      */
     @Input
     lateinit var interop: String
+
+    @Input
+    @Optional
+    var lib: String? = null
 }
 
 open class KonanLinkTest : KonanStandaloneTest() {