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/distinctBy.kt

@@ -0,0 +1,14 @@
+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() {
+    val sequence = fibonacci().dropWhile { it < 10 }.take(20)
+    assertEquals(listOf(13, 34, 55, 144), sequence.distinctBy { it % 4 }.toList())
+}