Add tests for graceful shutdown and fix related issues.

[ioquatix]

The initial graceful stop implementation was okay, but has some issues that will be revealed by extensive testing.

Types of Changes

• Bug fix.
• Maintenance.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.