Speed up download unit tests

These 2 tests are taking a lot more time than all the other tests combined:

```
tests/test_taskcluster.py::test_download_artifact_forbidden PASSED       [ 98%]
tests/test_taskcluster.py::test_download_artifact_badzip PASSED          [ 99%]
```

There is certainly a timeout / retry going haywire in the unit test context (no need to retry nor wait a lot here).