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/flatMap.kt

@@ -0,0 +1,8 @@
+import kotlin.test.*
+
+import kotlin.comparisons.*
+
+fun box() {
+    val result = sequenceOf(1, 2).flatMap { (0..it).asSequence() }
+    assertEquals(listOf(0, 1, 0, 1, 2), result.toList())
+}