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/substringDelimited.kt

@@ -0,0 +1,27 @@
+import kotlin.test.*
+
+
+
+fun box() {
+    val s = "-1,22,3+"
+    // chars
+    assertEquals("22,3+", s.substringAfter(','))
+    assertEquals("3+", s.substringAfterLast(','))
+    assertEquals("-1", s.substringBefore(','))
+    assertEquals("-1,22", s.substringBeforeLast(','))
+
+    // strings
+    assertEquals("22,3+", s.substringAfter(","))
+    assertEquals("3+", s.substringAfterLast(","))
+    assertEquals("-1", s.substringBefore(","))
+    assertEquals("-1,22", s.substringBeforeLast(","))
+
+    // non-existing delimiter
+    assertEquals("", s.substringAfter("+"))
+    assertEquals("", s.substringBefore("-"))
+    assertEquals(s, s.substringBefore("="))
+    assertEquals(s, s.substringAfter("="))
+    assertEquals("xxx", s.substringBefore("=", "xxx"))
+    assertEquals("xxx", s.substringAfter("=", "xxx"))
+
+}