Run tests against several servers and implement blocklist

[wm75]

Benefits:

1. ensures that iwc WFs are usable on more public Galaxy instances
2. helps discover differences in installed tools on usegalaxy.* instances

We could start by testing on usegalaxy.org and usegalaxy.eu.

WFs that are known to run only on a specific server could have an accompanying blocklist in their tests (one option could be in the form of comment lines in the test.yml file).

[jmchilton]

If we're doing this for PRs, it would be awesome to to extend it to "publishing" the latest revision of PR'd workflows to these servers for better interactive testing - e.g. create a "IWC Pull Request #210 - " workflow and keep it up to date and published as the PRs change. This is a response to a question from @bwlang at the GCC 2021 Americas workflow session.

"Meaningful IWC review involves downloading the .ga file, uploading to a local galaxy, then running locally. Any plans to reduce the barrier to review?"