Missing test that checks old releases are dropped after a deployment

[DavidMazarro]

While reviewing #174, @jpvillaisaza realized (see https://github.com/stackbuilders/hapistrano/pull/174#discussion_r800725192) that I had accidentally removed the call to dropOldReleases when a successful release was deployed, so old releases weren't being deleted; despite of this, my PR did pass the CI checks. This made clear that we need to add a test to check for that, as this behavior is to be expected since by default no more than 5 releases should be preserved unless the user increases that with the keep_releases setting via config.