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/append.kt

@@ -0,0 +1,9 @@
+import kotlin.test.*
+
+
+fun box() {
+    // this test is needed for JS implementation
+    assertEquals("em", buildString {
+        append("element", 2, 4)
+    })
+}