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/castGenericNull.kt

@@ -0,0 +1,13 @@
+fun <T> castToString(t: T) {
+    t as String
+}
+
+
+fun box(): String {
+    try {
+        castToString<Any?>(null)
+    } catch (e: Exception) {
+        return "OK"
+    }
+    return "Fail"
+}