Issue node shutdown on empty workload

[cybojanek]

Remote pytest_runtestloop requires at least 2 tests, or a test
and a shutdown command. In case of inter test locking, for
example using a file lock, the tests could deadlock, since the
shutdown command would not be enqueued, and the last test of a
worker would never finish, and would not allow another worker
to make progress.

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs:

• Make sure to include reasonable tests for your change if necessary

• We use towncrier for changelog management, so please add a news file into the changelog folder following these guidelines:

  • Name it $issue_id.$type for example 588.bugfix;

  • If you don't have an issue_id change it to the PR id after creating it

  • Ensure type is one of removal, feature, bugfix, vendor, doc or trivial

  • Make sure to use full sentences with correct case and punctuation, for example:

    Fix issue with non-ascii contents in doctest text files.
    

[cybojanek]

I'm not sure if calling node.shutdown() is safe the way I'm doing it, since there's a lot of logic to follow.

[nicoddemus]

I have used this test on master and indeed it does deadlock, and the PR fixes it. Good work!