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/smartCasts/whenSmartCast.kt

@@ -0,0 +1,9 @@
+fun baz(s: String?): Int {
+    if (s == null) return 0
+    return when(s) {
+        "abc" -> s
+        else -> "xyz"
+    }.length
+}
+
+fun box() = if (baz("abc") == 3 && baz("") == 3 && baz(null) == 0) "OK" else "FAIL"