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/MapTest/mapKeys.kt

@@ -0,0 +1,15 @@
+import kotlin.test.*
+
+
+fun <T> assertStaticTypeIs(value: T) {}
+fun box() {
+    val m1 = mapOf("beverage" to "beer", "location" to "Mells")
+    val m2 = m1.mapKeys { it.key + "2" }
+
+    assertEquals(mapOf("beverage2" to "beer", "location2" to "Mells"), m2)
+
+    val m1p: Map<out String, String> = m1
+    val m3 = m1p.mapKeysTo(mutableMapOf()) { it.key.length }
+    assertStaticTypeIs<MutableMap<Int, String>>(m3)
+    assertEquals(mapOf(8 to "Mells"), m3)
+}