[Tests] Migrate backend-independent tests from native to compiler/testData.

^KT-65979

compiler/testData/codegen/box/bridges/test18.kt

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the LICENSE file.
+ */
+// WITH_STDLIB
+
+import kotlin.test.*
+
+// overriden function returns Unit
+open class A {
+    open fun foo(): Any = 42
+}
+
+open class B: A() {
+    override fun foo(): Unit { }
+}
+
+fun box(): String {
+    val a: A = B()
+    val afoo = a.foo()
+    if (afoo != Unit) return "FAIL $afoo"
+    return "OK"
+}