Fix tests for mat with mat2

Our testsuite is using mat, that was repalced with mat2 and the commandline interface was changed to:

mat --show <file>.png

it creates a <file>.cleaned.png if run with:

mat <file>.png

Feature Branch: https://salsa.debian.org/tails-team/tails/merge_requests/9

Parent Task: #16281 (closed)

Related issues

• Blocks #16209

Original created by @Hefee on 16623 (Redmine)