Add "no_parallel" pytest's mark

[drew2a]

This PR fixes #6709 by introducing the no_parallel pytest's mark.

All tests that have this mark will be excluded from normal "parallel" Jenkins's run. But they will be run sequentially instead (like tunnel tests do).

In case this PR will be approved, I will merge it to the main and then modify corresponding Jenkins' jobs.

[sonarqubecloud]

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[drew2a]

The current Jenkins test run is the following:

pytest src/tribler-core -v --tx '12*popen//execmodel=eventlet' -p no:randomly --cov=tribler_core  --cov-report=xml:../output/coverage.xml --junitxml=test_report.xml

# Tunnel tests
pytest src/tribler-core/tribler_core/components/tunnel/tests/test_full_session --tunneltests --log-cli-level=INFO

pytest src/tribler-core -v -p no:randomly --no_parallel

@devos50 @kozlovsky do you know, is it possible to share coverage reports among the different test runs? It would be nice to include coverage from no_parallel and tunneltests to the main coverage report.

[devos50]

@drew2a yes, I believe cobertura has an option to merge separate code coverage files into one (but they need to have the same base path). We should merge the reports generated by the "no parallel" and parallel test runs. For the tunnel tests, however, I'm not so sure: the tunnel tests are more leaning to a full-fletched application test and the code coverage reported by them might hide lines that are uncovered by our unit tests.

[drew2a]

@devos50 ok, thanks. I'll find a way to merge coverage for parallel and no-parallel test runs.

[kozlovsky]

@drew2a it should be possible to use the --cov-append option.

Then the Jenkins command will look like:

pytest src/tribler-core -v --tx '12*popen//execmodel=eventlet' --cov=tribler_core  --cov-report=xml:../output/coverage.xml --junitxml=test_report.xml
pytest src/tribler-core -v --no_parallel --cov=tribler_core  --cov-report=xml:../output/coverage.xml --cov-append

[drew2a]

Jenkins jobs that run for a PR have been updated (Win, Linux, MacOS).

@devos50 @kozlovsky 🙏