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/unit/nullableUnit.kt

@@ -0,0 +1,20 @@
+fun isNull(x: Unit?) = x == null
+
+fun <T : Any> isNullGeneric(x: T?) = x == null
+
+fun deepIsNull0(x: Unit?) = isNull(x)
+fun deepIsNull(x: Unit?) = deepIsNull0(x)
+
+fun box(): String {
+    if (!isNull(null)) return "Fail 1"
+
+    val x: Unit? = null
+    if (!isNull(x)) return "Fail 2"
+
+    val y = x
+    if (!isNullGeneric(y)) return "Fail 3"
+
+    if (!deepIsNull(x ?: null)) return "Fail 4"
+
+    return "OK"
+}