backend/tests: Add blackbox tests from Kotlin JVM

Added tests from testData/codegen/box directory. There are blackbox tests
in other directories and they are to be added.

backend.native/tests/external/codegen/blackbox/casts/unitAsAny.kt

@@ -0,0 +1,11 @@
+// TODO: muted automatically, investigate should it be ran for JS or not
+// IGNORE_BACKEND: JS
+
+fun println(s: String) {
+}
+
+fun box(): String {
+    val x = println(":Hi!") as Any
+    if (x != Unit) return "Fail: $x"
+    return "OK"
+}