test: Sane test data download retries (#227)

### Description
The default number of retries of `multiurl.download` are very high:

https://github.com/ecmwf/multiurl/blob/develop/multiurl/http.py#L54-L55

In case of network errors, it will retry for 16 hours. Lower this to a
saner value. I believe this is one of the reasons the CI is sometimes
hanging for a long time.

I chose arbitrary new values, input welcome. 

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Agreement](https://github.com/ecmwf/codex/blob/main/Legal/contributor_license_agreement.md).
* The code follows the project's coding standards.
* I have performed self-review and added comments where needed.
* I have added or updated tests to verify that my changes are effective
and functional.
* I have run all existing tests and confirmed they pass.

Author: gmertes

src/anemoi/utils/testing.py

@@ -114,7 +114,7 @@ def __call__(self, path: str, gzipped: bool = False) -> str:
 
         LOG.info(f"Downloading test data from {url} to {target}")
 
-        download(url, target)
+        download(url, target, maximum_retries=5, retry_after=60)
 
         if gzipped:
             import gzip