Fix: remove TestObserver class

[coderimus]

Description

Hello,

The Magento\ProductAlert\Controller\Add\TestObserver, as I can see, was used for testing propose. It can run methods for sending product alert (price and stock) emails and, also, error emails for admin: Magento\ProductAlert\Model\Observer::process(). The main reason why I created this PR and remove reported class is that it can be run by any logged in customer while process() method should be run only by an event or by cron.

Looking forward to your reply,
Alex

Fixed Issues (if relevant)

N/A

Manual testing scenarios

1. Logged in to the frontend area using your customer account
2. Set the breakpoint for debugging in your IDE here: Magento\ProductAlert\Controller\Add\TestObserver::execute() line 18
3. Use next URL for test/debug: <base_url>/productalert/add/testObserver/
4. In browser you will see the blank page. In your debug console you will be stopped at the breakpoint and can continue with the process() method

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)