test: mock pypandoc.convert_text when testing export views

[afuetterer]

Description

Resolves #1092 and #1113

Tasks

• [x] add mocked_convert_text fixture, which should be used in combination with export_format fixture, when testing export views
• [x] add mocked_convert_text to all export view test functions
• [x] remove special treatment of XML export (e.g. assert root.tag == 'rdmo')
• [x] add a new parametrized "unit test", that tests all export formats with one input, see https://github.com/rdmorganiser/rdmo/pull/1117#issuecomment-2285990209
• [x] test for invalid export format
• [x] assert status codes
• [x] assert Content-Type header
• [x] assert Content-Disposition header
• [ ] squash commits

Types of Changes

• [x] Refactoring (no functional changes, no api changes)

Checklist

• [x] I have read the contributor guide.
• [x] My code follows the code style of this project.