No end-to-end test runs the diskern binary

[Muawiya-contact]

Every test in the workspace is a unit test inside the module it covers.
Nothing runs the diskern binary and checks what a user actually sees,
so the things only the binary does are unverified:

• argument parsing and the errors for bad input
• exit codes
• the grouping and ordering of printed findings
• --json producing parseable JSON with the fields the README promises

The CLI's own tests cover human_bytes and nothing else. A regression in
print_findings — say every finding printed under the wrong verdict
heading — passes CI today.

assert_cmd plus tempfile is the usual
shape:

#[test]
fn scan_reports_a_temp_file_it_was_pointed_at() {
    let dir = tempfile::tempdir().unwrap();
    std::fs::write(dir.path().join("big.dmg"), vec![0u8; 4096]).unwrap();

    Command::cargo_bin("diskern").unwrap()
        .args(["scan", dir.path().to_str().unwrap()])
        .assert()
        .success()
        .stdout(predicates::str::contains("Installers"));
}

Worth covering the --json path with serde_json::from_slice on stdout
rather than string matching, since that is the output other tools depend
on.

Keep fixtures inside a tempdir — scanning anything else from a test is
slow and machine-dependent.

crates/diskern-cli/src/main.rs:261

[LouisDeconinck]

I'd like to take this one — planning to add an assert_cmd + tempfile end-to-end test that runs the actual diskern binary against a fixture dir, covering the human output and the parseable --json path. Will open a PR shortly.