Use a masterKey for the tests

Currently, the tests only work with a MeiliSearch instance running without any master-key.

The tests should work with a MeiliSearch instance running with a master key named `masterKey`, launched this way:

```bash
$ docker run -it --rm -p 7700:7700 getmeili/meilisearch:v0.12.0 ./meilisearch  --no-analytics=true --master-key=masterKey
```