Fix robot tests to work with 'Image' from ba0c476

This is a tiny fix in robot tests to match the last commit https://github.com/will-moore/openmicroscopy/commit/ba0c4766df802f668ef257726194f297407867fe from #3207.

Should fix failures such as
```
08:56:21 Element 'xpath=//tr[contains(@class, 'data_heading_id')]/th[contains(text(), 'image')]' did not appear in 5 seconds
```