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/multiDecl/SimpleVarsExtensions.kt

@@ -0,0 +1,10 @@
+class A {
+    operator fun component1() = 1
+    operator fun component2() = 2
+}
+
+fun box() : String {
+    var (a, b) = A()
+    a = b
+    return if (a == 2 && b == 2) "OK" else "fail"
+}