fix: remove the tests' inclusion in the plugin

[MaferMazu]

Description

This PR removes the inclusion of the test as a dependency for the plugin and also changes the composer.json to map and autoload the test only in dev mode.

Context
We removed the addition of test files in a production plugin in #76, which generated problems because the test module was included in class-openedx-commerce.php. Problem: #62 (comment)

Testing instructions

1. Download this plugin with these changes: ZIP version, that was generated by this workflow.

2. Install and activate the plugin in a clean WordPress instance (E. g., You can use Docker or Local)

If you want to go further, you can configure the plugin and test other functions.

Additional information

The URL of the zip file I created for testing this PR has the tag v2.0.3, but it is irrelevant because the workflow uploads the zip file in the latest release of my repo which is using this updated branch.
Ref

Video demo (Using Local)

Installing and activating the plugin in a clean installation
https://github.com/openedx/openedx-wordpress-ecommerce/assets/35668326/e9fb881a-8095-473e-a6b4-6ce1a2acc3bb

Checklist for Merge

• Tested in a remote environment (clean and existent env)
• Rebased master/main
• Squashed commits

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[felipemontoya]

The changes look correct to me. Is what I would expect to solve the reported issue.

I downloaded the linked plugin package and it installs succesfully.

[MaferMazu]

I'll merge this and run the action to generate the ZIP in release 2.0.4 (because the first one wasn't working correctly for the issue this PR is solving).

[openedx-webhooks]

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