test(saltcheck): add test for cron envs (absent & present)

[myii]

PR progress checklist (to be filled in by reviewers)

• Changes to documentation are appropriate (or tick if not required)
• Changes to tests are appropriate (or tick if not required)
• Reviews completed

---

What type of PR is this?

Primary type

• [build] Changes related to the build system
• [chore] Changes to the build process or auxiliary tools and libraries such as documentation generation
• [ci] Changes to the continuous integration configuration
• [feat] A new feature
• [fix] A bug fix
• [perf] A code change that improves performance
• [refactor] A code change that neither fixes a bug nor adds a feature
• [revert] A change used to revert a previous commit
• [style] Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc.)

Secondary type

• [docs] Documentation changes
• [test] Adding missing or correcting existing tests

Does this PR introduce a BREAKING CHANGE?

No.

Related issues and/or pull requests

Completes #12 (CC: @SynPrime).

Describe the changes you're proposing

Add a saltcheck test that checks for environment variables that should be present and absent.

Pillar / config required to test the proposed changes

As added to the test pillar.

Debug log showing how the proposed changes work

https://travis-ci.com/saltstack-formulas/cron-formula/jobs/252449396#L1993-L2064

Documentation checklist

• Updated the README (e.g. Available states).
• Updated pillar.example.

Testing checklist

• Included in Kitchen (i.e. under state_top).
• Covered by new/existing tests (e.g. InSpec, Serverspec, etc.).
• Updated the relevant test pillar.

Additional context

[saltstack-formulas-travis]

🎉 This PR is included in version 0.3.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀