[WIP] ci: Add weekly test workflow file

[dashmage]

Runs lint, unit and functional tests at specified time weekly. If the test fails, the pre-configured Mattermost channel is notified with a message.

In order to understand the changes better, take a look at the runs in "Weekly Tests" action in the dashmage/charm-local-users project.

Run with test passing Run with test failing and Mattermost channel being notified

The above POC was configured with a webhook from my Mattermost user which would create a message like this:

Helpful references:

• failure() status check function
• Mattermost notify action
• schedule event to trigger workflow

Note: Status is WIP because we are still waiting for the Mattermost bot account through which we can create the webhook and add the MATTERMOST_WEBHOOK_URL secret`.

[dashmage]

Closing this PR since we're going ahead with the approach where all the tests are triggered from a single workflow