Support for running version mismatch tests in CI

[yadudoc]

Is your feature request related to a problem? Please describe.

In any context where funcx-endpoints are running containers, eg, in Kubernetes, there's a good chance that the python version and the funcx version installed in the container do not match the endpoint. Ideally, we should be testing this regularly in CI.

Describe the solution you'd like

There are some tests in #637 that are geared for being run manually that could be extended to get running in CI.
We do not run conda in CI, so we might have to rework those tests to use virtualenvs.

Tagging @benclifford since we'd had a chat about this.