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/classes/classObjectToString.kt

@@ -0,0 +1,9 @@
+// TODO: Enable for JS when it supports Java class library.
+// IGNORE_BACKEND: JS
+class SomeClass { companion object }
+
+fun box() = 
+    if ((SomeClass.toString() as java.lang.String).matches("SomeClass\\\$Companion@[0-9a-fA-F]+"))
+        "OK"
+    else
+        "Fail: $SomeClass"