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/slice.kt

@@ -0,0 +1,12 @@
+import kotlin.test.*
+
+
+
+fun box() {
+    val iter = listOf(4, 3, 0, 1)
+    // abcde
+    // 01234
+    assertEquals("bcd", "abcde".substring(1..3))
+    assertEquals("dcb", "abcde".slice(3 downTo 1))
+    assertEquals("edab", "abcde".slice(iter))
+}