Spurious test failures: user lookups not stable

[fabianfreyer]

The tests fail every once in a while, because the wrong UID is returned when looking up the nobody user.

The weird thing is, this only seems to happen in cargo test, but not when filtering out the exact test that fails: cargo test --lib tests::parse_rule.

[fabianfreyer]

see e.g.: https://cirrus-ci.com/task/6250330503249920:

failures:
---- tests::parse_subject stdout ----
thread 'tests::parse_subject' panicked at 'assertion failed: `(left == right)`
  left: `User(User(4294967295))`,
 right: `User(User(65534))`', src/lib.rs:1865:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
failures:
    tests::parse_subject
test result: FAILED. 11 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
error: test failed, to rerun pass '--lib'
Exit status: 101