test: Initial unit testing

- Test quests vector loading
- Test default quest on empty fields
- Test correct quest
- Fixed config paths handling

tests/cfg/quests/2.toml

@@ -0,0 +1,8 @@
+# This example demonstrates incorrect quest which will not be loaded from config
+# see main.rs
+id = 133713371337
+difficulty = sick
+reward = an infinite amount of coffee
+name = "Impossible quest"
+description = "This quest won't be loaded in the game, therefore you can't solve it"
+answer = 42