Add integration tests for the bot's Cog

[ctmbl]

This is a tracking issue created during #13 development.
Currently the library I use to test the bot, dpytest, isn't well documented nor finished so I have a few issues that prevent me from writing all the tests I'd like. I'll try to open some Issues/PR on the repo to address these or hopefully find some workarounds

[ctmbl]

I created CraftSpider/dpytest#114 and CraftSpider/dpytest#116 to deal with these problem but I finally find a workaround for CraftSpider/dpytest#116 and I'll be able to ENABLE the test I disabled in 17b39a0 (during #13 dev)