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/StringNumberConversionTest/toBoolean.kt

@@ -0,0 +1,8 @@
+import kotlin.test.*
+
+fun box() {
+    assertEquals(true, "true".toBoolean())
+    assertEquals(true, "True".toBoolean())
+    assertEquals(false, "false".toBoolean())
+    assertEquals(false, "not so true".toBoolean())
+}