Add tests documentation

docs/developer/TESTING.md

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+# Dangerzone Testing
+
+Dangerzone has some automated testing under `tests/`.
+
+The following assumes that you have already setup the development environment.
+
+## Run tests
+
+Unit / integration tests are run with:
+
+```bash
+poetry run make test
+```
+
+## Run large tests
+
+We also have a larger set of tests that can take a day or more to run, where we evaluate the completeness of Dangerzone conversions.
+
+```bash
+poetry run make test-large
+```
+
+### Test report generation
+After running the large tests, a report is stored under `tests/test_docs_large/results/junit/` and it is composed of the JUnit XML file describing the pytest run.
+
+This report can be analysed for errors. It is obtained by running:
+
+```bash
+cd tests/docs_test_large
+make report
+```
+
+If you want to run the report on some historical test result, you can call:
+
+```bash
+cd tests/docs_test_large
+python report.py tests/test_docs_large/results/junit/commit_<COMMIT_ID>.junit.xml
+```