add a bot command for testing the bot

[bedroge]

It would be nice if we can test the (main) bot functionality whenever we make changes (new functionality, bug fixes, etc) to the code of the bot. For instance, PR EESSI/software-layer#488 introduced some bugs (because it only tested the rebuild functionality) and EESSI/software-layer#518 tried to fix these, but in order to test it we have to make additional PRs based on that one with some (dummy) easystack to test it. We could consider adding a command like bot: test (or buildtest), which will trigger the bot to just create some dummy easystack(s) itself that it can use to do a few test (re)builds.