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/plusAssign.kt

@@ -0,0 +1,21 @@
+import kotlin.test.*
+import kotlin.comparisons.*
+
+fun box() {
+    // lets use a mutable variable of readonly list
+    var l: List<String> = listOf("cheese")
+    val lOriginal = l
+    l += "foo"
+    l += listOf("beer")
+    l += arrayOf("cheese", "wine")
+    l += sequenceOf("bar", "foo")
+    assertEquals(listOf("cheese", "foo", "beer", "cheese", "wine", "bar", "foo"), l)
+    assertTrue(l !== lOriginal)
+
+    val ml = arrayListOf("cheese")
+    ml += "foo"
+    ml += listOf("beer")
+    ml += arrayOf("cheese", "wine")
+    ml += sequenceOf("bar", "foo")
+    assertEquals(l, ml)
+}