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/privateConstructors/withVarargs.kt

@@ -0,0 +1,13 @@
+// See also KT-6299
+public open class Outer private constructor(val s: String, vararg i: Int) {
+    class Inner: Outer("xyz")
+    class Other: Outer("abc", 1, 2, 3)
+    class Another: Outer("", 42)
+}
+
+fun box(): String {
+    val outer = Outer.Inner()
+    val other = Outer.Other()
+    val another = Outer.Another()
+    return "OK"
+}