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/constructors.kt

@@ -0,0 +1,24 @@
+import kotlin.test.*
+
+
+fun box() {
+    StringBuilder().let { sb ->
+        assertEquals(0, sb.length)
+        assertEquals("", sb.toString())
+    }
+
+    StringBuilder(16).let { sb ->
+        assertEquals(0, sb.length)
+        assertEquals("", sb.toString())
+    }
+
+    StringBuilder("content").let { sb ->
+        assertEquals(7, sb.length)
+        assertEquals("content", sb.toString())
+    }
+
+    StringBuilder(StringBuilder("content")).let { sb ->
+        assertEquals(7, sb.length)
+        assertEquals("content", sb.toString())
+    }
+}