Check we're not in an xdist worker before submitting payload

[joshprice]

The way xdist works is somewhat surprising:

• one might expect a single pytest execution, however all workers run the full pytest lifecycle as well as the main controller process resulting in n+1 pytest executions (where n is the actual number of workers)
• the workers also swallow stdout so introspecting this was producing misleading results when trying to reproduce locally (see this thread for more info https://github.com/pytest-dev/pytest-xdist/issues/354)

This checks that we're in the controller process (eg the existence of "workerinput" implies an xdist worker thread), and only submits the results payload if we're in the controller or master process.

The JUnit test reporter has a range of other strategies to deal with xdist but this particular check inspired this fix https://github.com/pytest-dev/pytest/blob/ac4189875562a6ce5b502c3dd87156c0367cf8f1/src/_pytest/junitxml.py#L429

[joshprice]

Thanks @pda, I've verified using this repo https://github.com/team-alembic/pytest-bk-test-analytics which was a fork of https://github.com/adimoldovan/pytest-bk-test-analytics it'd be great if @adimoldovan can verify the fix on his repo.

I'd like to ensure we've got the most robust possible fix here with tests as well before we publish a new version.

Fixed no duplicates on the xdist-duplicates branch:

Duplicates on current version:

[adimoldovan]

I verified this, it looks good. Thanks for the fix @joshprice

[joshprice]

@pda this should be good for a patch release