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/constants/constantsInWhen.kt

@@ -0,0 +1,18 @@
+fun test(
+        b: Boolean,
+        i: Int
+) {
+    if (b) {
+        when (i) {
+            0 -> foo(1)
+            else -> null
+        }
+    } else null
+}
+
+fun foo(i: Int) = i
+
+fun box(): String {
+    test(true, 1)
+    return "OK"
+}