Unit tests: Add mock network tests and more thorough tests of `Client`

[clement-leprovost]

Especially for the Query class.

[clement-leprovost]

Done in the v3 branch:

• Query class
• BrowseIterator

Still missing:

• Mock network tests (to thoroughly test the error handling and retry logic)
• Client class

Note: testing request cancellation is impossible due to the nature of the unit test framework: asynchronous methods get executed synchronously, so we have no chance to cancel them before they are run.