Repair pytest on GitLab CI

Since fc91cbd8, pytest fails on GitLab CI. Locally, everything works fine for me. This is the first job failing for this commit:

$ tools/tests/test-pytest.sh
============================= test session starts ==============================
platform linux -- Python 3.7.3, pytest-5.2.0, py-1.8.0, pluggy-0.13.0
rootdir: /builds/nicoalt/briar-gtk, inifile: pytest.ini
plugins: cov-2.7.1, mock-1.11.0, requests-mock-1.7.0
collected 20 items

tests/briar/api/test_constants.py ....                                   [ 20%]
tests/briar/api/models/test_contacts.py .                                [ 25%]
tests/briar/api/models/test_private_chat.py ..                           [ 35%]
tests/briar/gtk/test_application.py .....tools/tests/test-pytest.sh: line 6:  4406 Trace/breakpoint trap   (core dumped) PYTHONPATH=src pytest --cov=src tests/
ERROR: Job failed: exit code 1

Until this is fixed, I allowed the pipeline to fail with d519ebc7.