Build training provider URLs using Hash#to_query

What does this MR do and why?

Construct the URL query strings using a parameterized method instead of string concatenation. This is more maintainable, and is less error prone. Additionally, use stub_requests in tests instead of stubbing the return value of try_get. This is slightly more accurate for imitating HTTP responses, and lets us assert that the request to the training provider is correctly formed.

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