Enable configuring numprocesses's default `tx` command

Over the years I've used and introduced xdist whenever possibly to speed up pytest runs.

Usually just using the -n X notation was sufficient. But in our current application, we have to use the --tx notation to ensure we're using eventlet.

--tx '4*popen//execmodel=eventlet'

This is a lot to type if you 'just' want to speed up tests. And combining it with -n reverts it to just using popen.
Ideally I'd configuring it to default to using popen//execmodel=eventlet and then up the processes using -n X notation.

So my feature request would be:

Enable configuring what 'executing method' -n actually uses. With popen being the default.

So that in your pytest.ini you can do something like this:

[pytest]
addopts = --default-tx popen//execmodel=eventlet

And then can add more worker as desired with the -n notation.

Start by tracing how pytest-xdist handles the -n and --tx options, including values supplied through pytest.ini addopts. Define the configuration for a default execution method, preserve popen as the default, and verify that -n X creates the requested number of workers using that method.