Add doc for unit testing (phpunit)

[PtyMatt]

Description of your feature

In the Tenant Aware or Tutorials section of the docs, there should be an article explaining how to make tests "tenant aware".

Current behavior

This complete but outdated tuto (v5.1) explains very well how to make tests "tenant aware".
But since the tenancy:install artisan command no longer exists, this doesn't work anymore.

Issues like #640 or #627 (comment) doesn't really help me, I'm not an expert of this package nor phpunit config!

Information

• hyn/multi-tenant version: 5.3
• laravel version: 5.7
• php: 7.2.11
• MariaDB: 10.2.18

[PtyMatt]

@luceos do you have any schedule or milestone for this doc?

I will be willing to give any feedback on it :)

[luceos]

do you have any schedule or milestone for this doc?

Not really no.

Feel free to send in a PR if you want.

[PtyMatt]

Closing it as this pull request is almost answering what I need :)