Allow re-use of an existing xpython project #103

What is the problem or limitation you are having?

At present, xpython is a "one-and-done" command - it encompasses both the clone and run parts of testbed usage. It would be useful to be able to re-use an xpython environment once it has been configured.

Describe the solution you'd like

At present, xpython --work-dir <dir> allows the user to specify a location for the working directory where a testbed project will be cloned. However, if the directory already exists, xpython raises an error.

We should modify xpython so that if the working directory already exists, it will re-use the existing project. The project will not be re-cloned; sources and requirements will not be re-installed. Any arguments dependencies or sources will be ignored; the existing project will be used as-is, but with the new arguments being passed to the runner.

Describe alternatives you've considered

Leave xpython as one-and-done, requiring rebuilding the testbed for every command.

Additional context

No response