Prepare rsvg-convert tests for the rustified version
- Some minor cleanups to the rsvg-convert testsuite
- Relax the tests with regard to error messages and output for the --help and --version options.
- Change rsvg-convert to reject invalid resolutions, and adjust the tests accordingly.
This prepares the codebase for the merge of the rustified version of rsvg-convert. The idea is that the new binary should pass all the tests that the legacy C binary passed. This required some smaller adjustments to the tests as I do not want to have to reproduce error messages and other output perfectly.