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/average.kt

@@ -0,0 +1,12 @@
+import kotlin.test.*
+import kotlin.comparisons.*
+
+fun box() {
+    assertTrue { arrayListOf<Int>().average().isNaN() }
+    expect(3.8) { listOf(1, 2, 5, 8, 3).average() }
+    expect(2.1) { sequenceOf(1.6, 2.6, 3.6, 0.6).average() }
+    expect(100.0) { arrayListOf<Byte>(100, 100, 100, 100, 100, 100).average() }
+    val n = 100
+    val range = 0..n
+    expect(n.toDouble() / 2) { range.average() }
+}