Increase coverage

[gecko0307]

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