Add tests

[FossoresLP]

A DNS server is a mission critical part of the network infrastructure so it should be tested very well. I'm working on that but it will take time to add tests for all parts of this project as some things are not easy to test with normal unit tests. I will therefore write integration tests but those take even more time to write.

Tests to write

• Header parsing [unit] (Added in commit 83c71cb)
• Label parsing [unit] (Added in commit 78fb456)
• Query parsing [unit] (Added in commit 2713883)
• Response parsing [unit] (Added in commit d78f1a5)
• Message parsing [unit]
• Cache [integration]
• DNS resolution [integration]
• Zone file parsing [integration]