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/stringBuild.kt

@@ -0,0 +1,10 @@
+import kotlin.test.*
+
+
+fun box() {
+    val s = buildString {
+        append("a")
+        append(true)
+    }
+    assertEquals("atrue", s)
+}