add pytest integration tests

Use pytest to manage background services, paths, and environment
variables.

Benefits:
- Tests are a little easier to write.
- Cleanup is more reliable. You can CTRL-C a test and it will still shut
  down gracefully. If you manually start a conflicting process, the test
  fixtures will detect this and abort at startup.
- Don't need to worry about remembering '--test-threads=1'
- Output of sub-processes can be captured to files.
- Test fixtures configure everything to operate under a single test
  output directory, making it easier to capture logs in CI.
- Detects all the necessary paths if run from the git root, but can also
  run from arbitrary paths by setting environment variables.

There is also a deliberately broken test (test_broken.py) that can be
used to test whether the test fixtures properly clean up after
themselves. It won't run by default; the comment at the top explains how
to enable it.

.gitignore

@@ -2,5 +2,7 @@
 /tmp_check
 /tmp_install
 /tmp_check_cli
+__pycache__/
+test_output/
 .vscode
 .zenith