Merge #514: Add announce field to test torrent

f64cb29 test: add announce field to test torrent (Jose Celano)

Pull request description:

  Add `announce` field to test torrent.

  With the command:

  ```
  cargo run --bin create_test_torrent ./output/test/torrents
  ```

  You can generate a test torrent. Currently, it does now add all the optional fields. We are adding them when we need them.

ACKs for top commit:
  josecelano:
    ACK f64cb29

Tree-SHA512: a5b7b143f8e73651d2612920e75715ae4c616fd719f82a2156186939b909e49d411d388381479cb3bb3dff9381acb6be526dbb3cf7d3635b0376f1806e21f601

src/bin/create_test_torrent.rs

@@ -42,7 +42,7 @@ fn main() {
                 md5sum: None, // DevSkim: ignore DS126858
             }],
         ),
-        announce: None,
+        announce: Some("https://tracker.torrust-demo.com/announce".to_string()),
         nodes: Some(vec![("99.236.6.144".to_string(), 6881), ("91.109.195.156".to_string(), 1996)]),
         encoding: None,
         httpseeds: Some(vec!["https://seeder.torrust-demo.com/seed".to_string()]),