Make tests more predictable by publishing default content stubs

[caendesilva]

Adding a todo:

To make testing more predictable, I want to create stubs for the content files that should be included in Hyde installs.
These would be:

• _pages\index.blade.php
• _pages\404.blade.php
• _site\media\app.css

Before and after each test all content directories are deleted, and createsdefaultdirectories are run, and stubs republished.

The backup file helpers can then also be removed, as tests should never be run in a production environment.
Edit: Keeping the file helpers as I'm keeping the backup/restores of other files that are not in the scope of content files, such as config files etc.

Originally posted by @caendesilva in #181 (comment)

[caendesilva]

The contents of this issue has been implemented, however they add several seconds to the test time.

[caendesilva]

Fixed it so it is actually faster than before.