Translate Mango Python integration tests to Elixir

Currently, Mango has its own integration tests that are implemented in Python and have to be activated separately via the `mango-tests` `make(1)` target. Re-write them in Elixir to integrate them better with the rest of the integration tests that are in use for CouchDB. This move could bring more simplification and regularization to the code base, removing the dependency on Nose2.

Start with the existing Mango integration tests activated by the `mango-tests` make target, and review how they depend on Python and Nose2. Compare them with CouchDB's other Elixir integration tests to define the migration boundary. Done means the Mango tests run as part of the regular Elixir integration-test workflow without the separate Nose2 dependency.

Written by the indexing model from the issue text.