ci: Add a test dependency

Add libqt5gui5 as a test dependency in the 'convert-test-docs' step.
This package brings several other Qt and graphics libraries, which are
the ones that we actually require to run the tests *with PySide6*. Else,
we encounter this error:

```
Traceback (most recent call last):
  File "/home/circleci/project/dangerzone/gui/__init__.py", line 19, in <module>
    from PySide6 import QtCore, QtGui, QtWidgets
ImportError: libEGL.so.1: cannot open shared object file: No such file or directory
```

Note that the same package is not required when importing PySide2.QtGui,
which is why we hadn't encountered this issue before. Also, in the rest
of our environments, we explicitly install libqt5gui5, in order to run
the Dangerzone GUI.

.circleci/config.yml

@@ -155,6 +155,10 @@ jobs:
             # This flag is important, due to an open upstream Poetry issue:
             # https://github.com/python-poetry/poetry/issues/7184
             poetry install --no-ansi
+      - run:
+          name: Install test dependencies
+          command: |
+            sudo apt-get install -y libqt5gui5 --no-install-recommends
       - run:
           name: Prepare cache directory
           command: |