Clean up integration tests

[lihaoyi]

• Now that curl uses --retry-all-errors we can remove the explicit sleeps

• Remove the serverId file as part of example/integration test cleanup to try and force a shutdown if the mill shutdown call doesn't work for some reason

  • I tried making the ./mill shutdown command after each integration test either required to succeed or removing it.
    • Making it required to succeed does not work because if the Mill build.mill fails to compile the shutdown task is not available and it fails, and we have a lot of integration tests that fail to compile since that's what they're testing
    • Removing it could work, but better to try our best to clean up the integration test subprocesses so others can run without interference

• Make ./mill shutdown return exit code 0 correctly, rather than letting it leak the exit code of the previously run command

• Mill server avoids logging anything when out/ folder is missing, to avoid re-creating the out-folder and potentially racing with whoever deleted it

Added some unit and integration tests to exercise the various codepaths