Programmatically get providers for tests

[danielfromearth]

As discussed in #460, @asteiker and I were wondering if we could move away from a manually curated list of providers, towards programmatically determining the providers, e.g., in https://github.com/nsidc/earthaccess/blob/14c4fbeb8f4d27bf9145b558684f7b501214de78/tests/integration/test_cloud_download.py#L15