Provide unit test coverage around multiple transfer attempts

[bbockelm]

In 7.6.x, we broke the ability to return multiple attempts in the plugin output. This is because we set the incorrect error object.

The fixes are in #1063 (but that came right up against the release day); this issue is for the regression test coverage.

We should check the following are done:

1. Ensure that, on success, the number of attempts is non-zero.
2. Force the transfer to take at least a second then ensure the total transfer time is >1s.
3. Register two caches (both httptest servers, not actual "federation in a box" caches) for a test federation. Have the first download always fail and second succeed. Check attempts and the developer data in the plugin.
4. Same as above but make both downloads fail. Check the attempts and error messages.