Add coverage

.gitlab-ci.yml

+image: debian
+
 test:
   script:
   - apt-get update -qy
-  - apt-get install -qy --no-install-recommends python3-mutagen python3-gi-cairo gir1.2-poppler-0.18 gir1.2-gdkpixbuf-2.0 libimage-exiftool-perl
-  - python3 -m unittest discover -v
+  - apt-get install -qy --no-install-recommends python3-mutagen python3-gi-cairo gir1.2-poppler-0.18 gir1.2-gdkpixbuf-2.0 libimage-exiftool-perl python3-coverage
+  - python3-coverage run -m unittest discover -s tests/
+  - python3-coverage report -m --include 'src/*'