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/StringTest/drop.kt

@@ -0,0 +1,12 @@
+import kotlin.test.*
+
+
+
+fun box() {
+    val data = "abcd1234"
+    assertEquals("d1234", data.drop(3))
+    assertFails {
+        data.drop(-2)
+    }
+    assertEquals("", data.drop(data.length + 5))
+}