Restore __tests__ folder in HelloWorld template

PR facebook#18019 Removed __tests__ and __fixtures__ folders from npm package.
But it also effectively removed __tests__ folder from initial project
template. This commit restores __tests__ in template application.

It applicable only to 0.58-stable branch because in master, templates
was moved to another location. I think we should recheck what is ignored
in master after this landed.

local-cli/.npmignore

@@ -1,2 +1,3 @@
 __fixtures__
 __tests__
+!/templates/HelloWorld/__tests__