Add tests for Excel/XSLX ingest

[landreev]

Model after the existing CSV test.

(this should be the first step, before re-visiting and improving the XSLX ingest plugin)

[cmbz]

To focus on the most important features and bugs, we are closing issues created before 2020 (version 5.0) that are not new feature requests with the label 'Type: Feature'.

If you created this issue and you feel the team should revisit this decision, please reopen the issue and leave a comment.