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/countBy.kt

@@ -0,0 +1,13 @@
+import kotlin.test.*
+
+
+fun box() {
+    val map = mapOf(Pair("b", 3), Pair("c", 2), Pair("a", 2))
+    assertEquals(3, map.count())
+
+    val filteredByKey = map.count { it.key == "b" }
+    assertEquals(1, filteredByKey)
+
+    val filteredByValue = map.count { it.value == 2 }
+    assertEquals(2, filteredByValue)
+}