Increase coverage

[gecko0307]

• [ ] dlib.audio - 0%
• [x] dlib.coding - 80%
• [x] dlib.concurrency - 96%
• [x] dlib.container - 94%
• [x] dlib.core - 74%
• [ ] dlib.filesystem - 41%
• [ ] dlib.geometry - 56%
• [ ] dlib.image - 14%
• [x] dlib.math - 92%
• [x] dlib.memory - 82%
• [ ] dlib.network - 60%
• [x] dlib.serialization - 79%
• [x] dlib.text - 68%