Catalog: Add unit tests for Cron classes

[dmytro-ch]

Description

This PR adds missing unit tests for the following classes:
\Magento\Catalog\Test\Unit\Cron\AvailabilityCheckerTest
\Magento\Catalog\Test\Unit\Cron\DeleteOutdatedPriceValuesTest

Fixed Issues (if relevant)

N/A

Manual testing scenarios

N/A

[magento-engcom-team]

Hi @dmytro-ch. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
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[magento-engcom-team]

Hi @dmytro-ch. Thank you for your contribution.
We will aim to release these changes as part of 2.2.7.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.