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/SetOperationsTest/plusAssign.kt

@@ -0,0 +1,21 @@
+import kotlin.test.*
+
+
+fun box() {
+    // lets use a mutable variable
+    var set = setOf("a")
+    val setOriginal = set
+    set += "foo"
+    set += listOf("beer", "a")
+    set += arrayOf("cheese", "beer")
+    set += sequenceOf("bar", "foo")
+    assertEquals(setOf("a", "foo", "beer", "cheese", "bar"), set)
+    assertTrue(set !== setOriginal)
+
+    val mset = mutableSetOf("a")
+    mset += "foo"
+    mset += listOf("beer", "a")
+    mset += arrayOf("cheese", "beer")
+    mset += sequenceOf("bar", "foo")
+    assertEquals(set, mset)
+}