QGIS docker tests fix coverage

[visr]

In #2862 I saw this coverage failure:

+ docker exec -t qgis sh -c qgis_setup.sh ribasim_qgis
Plugin master folder linked in /root/.local/share/QGIS/QGIS3/profiles/default/python/plugins/ribasim_qgis
+ echo Containers are running
Containers are running
+ docker exec -t -e QGIS_TEST_MODULE=ribasim_qgis.tests qgis sh -c cd /tests_directory/ribasim_qgis/tests && xvfb-run -a qgis --nologo --code /tests_directory/ribasim_qgis/scripts/qgis_testrunner.py
Warning: QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
QGIS Test Runner Inside - starting the tests ...
QGIS Test Runner - Trying to import ribasim_qgis.tests
QGIS Test Runner Inside - executing function <function run_all at 0x7f85f72da980>
test_plugin (test_load_plugin.TestPlugin.test_plugin)
Triggers Ribasim button and checks that Dock is added. ... ok
test_plugin_is_loaded (test_load_plugin.TestPlugin.test_plugin_is_loaded)
Test plugin is properly loaded and appears in QGIS plugins. ... ok
test_get_database_path_from_model_file (test_model.TestModel.test_get_database_path_from_model_file)
Tests that get_database_path_from_model_file() can find the input directory and appends the database.gpkg to it. ... ok
test_get_directory_path_from_model_file (test_model.TestModel.test_get_directory_path_from_model_file)
Tests that get_directory_path_from_model_file() can resolve paths from a toml file. ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.008s

OK
ERROR:root:QGIS Test Runner Inside - [FAILED] Exception: No data to report.
Traceback (most recent call last):
  File "<string>", line 121, in __run_test
  File "/root/.local/share/QGIS/QGIS3/profiles/default/python/plugins/ribasim_qgis/tests/__init__.py", line 20, in run_all
    cov.xml_report(outfile=testfolder / "coverage.xml")
  File "/usr/lib/python3/dist-packages/coverage/control.py", line 1210, in xml_report
    return render_report(self.config.xml_output, XmlReporter(self), morfs, self._message)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/coverage/report_core.py", line 61, in render_report
    ret = reporter.report(morfs, outfile=outfile)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/coverage/xmlreport.py", line 105, in report
    for fr, analysis in get_analysis_to_report(self.coverage, morfs):
  File "/usr/lib/python3/dist-packages/coverage/report_core.py", line 96, in get_analysis_to_report
    raise NoDataError("No data to report.")
coverage.exceptions.NoDataError: No data to report.
Error: Process completed with exit code 1.

which was bypassed by 945b443

We should make that work.