Add Screenshot Tests for the new Unit modes in the Spectrum Viewer

[MikeSullivan7]

Desired Behaviour

Currently, the Spectrum Viewer takes screenshots for example data when the spectrum is in the Image Index mode (i.e. the default). Some artificial spectrum data should be used so that checks can also be done for the other unit modes: Wavelength, Energy, and Tof us. This will make sure that the conversions are behaving properly when other changes are made in the future.,

Current Behaviour

Screenshot tests are only done for the Image Index mode in the Spectrum Viewer.