https://github.com/systemd/systemd/blob/master/.github/CONTRIBUTING.md
+When adding new functionality, tests should be added. For shared functionality
+(in src/basic and src/shared) unit tests should be sufficient. The general
+policy is to keep tests in matching files underneath src/test,
+e.g. src/test/test-path-util.c contains tests for any functions in
+src/basic/path-util.c. If adding a new source file, consider adding a matching
+test executable. For features at a higher level, tests in src/test/ are very
+strongly recommended. If that is no possible, integration tests in test/ are
+encouraged.
+
Please always test your work before submitting a PR. For many of the components
of systemd testing is straight-forward as you can simply compile systemd and
run the relevant tool from the build directory.