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/when/stringOptimization/nullability.kt

@@ -0,0 +1,43 @@
+// WITH_RUNTIME
+
+import kotlin.test.assertEquals
+
+fun foo1(x : String?) : String {
+    when (x) {
+        "abc", "cde" -> return "abc_cde"
+        "efg", "ghi", null -> return "efg_ghi"
+    }
+
+    return "other"
+}
+
+fun foo2(x : String?) : String {
+    when (x) {
+        "abc", "cde" -> return "abc_cde"
+        "efg", "ghi" -> return "efg_ghi"
+        else -> return "other"
+    }
+}
+
+fun box() : String {
+    //foo1
+    assertEquals("abc_cde", foo1("abc"))
+    assertEquals("abc_cde", foo1("cde"))
+    assertEquals("efg_ghi", foo1("efg"))
+    assertEquals("efg_ghi", foo1("ghi"))
+    assertEquals("efg_ghi", foo1(null))
+
+    assertEquals("other",   foo1("xyz"))
+
+    //foo2
+    assertEquals("abc_cde", foo2("abc"))
+    assertEquals("abc_cde", foo2("cde"))
+    assertEquals("efg_ghi", foo2("efg"))
+    assertEquals("efg_ghi", foo2("ghi"))
+
+
+    assertEquals("other",   foo2("xyz"))
+    assertEquals("other",   foo2(null))
+
+    return "OK"
+}