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/text/StringBuilderTest/asCharSequence.kt

@@ -0,0 +1,17 @@
+import kotlin.test.*
+
+
+fun box() {
+    val original = "Some test string"
+    val sb = StringBuilder(original)
+    val result = sb.toString()
+    val cs = sb as CharSequence
+
+    assertEquals(result.length, cs.length)
+    assertEquals(result.length, sb.length)
+    for (index in result.indices) {
+        assertEquals(result[index], sb[index])
+        assertEquals(result[index], cs[index])
+    }
+    assertEquals(result.substring(2, 6), cs.subSequence(2, 6).toString())
+}