Use a TrueType font for plotting sequence names with last/dotplot.

[charles-plessy]

The last-dotplot command normally picks scalable fonts by default but did not manage to do so on the conda install in the images. Therefore I hardcode the path (which is likely to change when the images are updated).

Not using a scalable font prevents users from changing the font size using the command-line argument -s of last-dotplot.

In addition, I found that the CI tests did not check the outputted images, and corrected that too in this commit.

PR checklist

Closes #XXX

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[charles-plessy]

Thanks @mashehu but the failing tests on conda revealed a real problem that I do not know how to solve now… I asked the following question on Slack:

I need to access a file that is in /opt/conda/pkgs/pillow-11.1.0-py313h8db990d_0/info/test/Tests/fonts/FreeMono.ttf in the Docker and Singularity images, but apparently it is elsewhere when using the conda runner. I have never used conda directly. How can I find the path to that file ?