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/collections/SequenceTest/mapIndexedNotNull.kt

@@ -0,0 +1,17 @@
+import kotlin.test.*
+
+import kotlin.comparisons.*
+
+fun fibonacci(): Sequence<Int> {
+    // fibonacci terms
+    // 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, ...
+    return generateSequence(Pair(0, 1), { Pair(it.second, it.first + it.second) }).map { it.first }
+}
+
+fun box() {
+    // find which terms are divisible by their index
+    assertEquals(listOf("1/1", "5/5", "144/12", "46368/24", "75025/25"),
+            fibonacci().mapIndexedNotNull { index, value ->
+                if (index > 0 && (value % index) == 0) "$value/$index" else null
+            }.take(5).toList())
+}