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/genericIntersection.kt

@@ -0,0 +1,9 @@
+// See also KT-7801
+class A
+
+fun <T> test(v: T): T {
+    val a: T = if (v !is A) v else v
+    return a
+}
+
+fun box() = test("OK")