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/CollectionTest/foldIndexed.kt

@@ -0,0 +1,19 @@
+import kotlin.test.*
+import kotlin.comparisons.*
+
+fun box() {
+    expect(42) {
+        val numbers = listOf(1, 2, 3, 4)
+        numbers.foldIndexed(0) { index, a, b -> index * (a + b) }
+    }
+
+    expect(0) {
+        val numbers = arrayListOf<Int>()
+        numbers.foldIndexed(0) { index, a, b -> index * (a + b) }
+    }
+
+    expect("11234") {
+        val numbers = listOf(1, 2, 3, 4)
+        numbers.map { it.toString() }.foldIndexed("") { index, a, b -> if (index == 0) a + b + b else a + b }
+    }
+}