Avoid private api

[nyalldawson]

The test https://github.com/qcooperative/qgis-core-tests/blob/master/coretests/tests/testerplugin.py#L59 relies on non-stable, private api here: https://github.com/qcooperative/qgis-core-tests/blob/master/coretests/tests/testerplugin.py#L123

This should be removed, as it could validly break in any future qgis release (it's not stable api).

More generally, I don't think it's a good idea to try to automate parts of the test like opening the dialog for users. To me this needlessly restricts the actual scope of test -- instead of the batch test also covering things like testing that users can search for an algorithm in the toolbox and testing that selecting the batch option correctly opens the batch dialog without issue, we're instead skipping a bunch of these useful tests! So by trying to be helpful, we're ending up with a less valuable test all round...

For this reason, I'd suggest we ONLY automate steps which are absolutely critical, and leave everything else as manual steps for users to do themselves.