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/joinToString.kt

@@ -0,0 +1,12 @@
+import kotlin.test.*
+import kotlin.comparisons.*
+
+fun box() {
+    val data = listOf("foo", "bar")
+    val text = data.joinToString("-", "<", ">")
+    assertEquals("<foo-bar>", text)
+
+    val mixed = listOf('a', "b", StringBuilder("c"), null, "d", 'e', 'f')
+    val text2 = mixed.joinToString(limit = 4, truncated = "*")
+    assertEquals("a, b, c, null, *", text2)
+}