tests: Copy stdlib tests from Kotlin/JVM

This patch copies Kotlin/JVM stdlib tests (libraries/stdlib/test)
excepting ones with explicit java dependencies. It also transforms
the tests to use them as box-tests.

backend.native/tests/external/stdlib/text/StringTest/orEmpty.kt

@@ -0,0 +1,11 @@
+import kotlin.test.*
+
+
+
+fun box() {
+    val s: String? = "hey"
+    val ns: String? = null
+
+    assertEquals("hey", s.orEmpty())
+    assertEquals("", ns.orEmpty())
+}