Clean up e2e test for linux

On Linux, need to use shell=True, and whole commands instead of parts. There are a bunch of conditional tests for Linux, that can be simplified.

- [ ] Update make_sp_call() in core e2e test utils to accept use_parts arg.
- [ ] Simplify existing checks about Windows, heroku, etc. make_sp_call() should not do any of that inspection.
- [ ] Simplify calls in e2e test utils for this plugin with the updated core test utility function.
- [ ] See also [core issue 405](https://github.com/django-simple-deploy/django-simple-deploy/issues/405), about removing use of `shell=True`.

Start with make_sp_call() in the core e2e test utilities, then inspect the plugin's e2e test utility calls that use it. Review core issue 405 for the shell=True context, update the utility and simplify the Linux, Windows, and Heroku checks described in the checklist. Run the e2e tests to confirm the revised command handling works.

Written by the indexing model from the issue text.