Add some tests for ExportManager::formatFilename

Summary: This adds some tests for formatFilename to ensure that by for example changing placeholders or working on the parsing logic we don't break anything.

Test Plan: Run the tests.

Reviewers: #spectacle, ngraham

Reviewed By: #spectacle, ngraham

Subscribers: ngraham, #spectacle

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D19969