Revise test_updater_ng.py

[sechkova]

Description of issue or feature request:

Currently test_updater_ng.py is the single place where ngclient/updater.py is tested with an HTTP server instance serving files from the file system and not with the in-memory files served by RepositorySimulator. The tests included are not well thought out and were added with the goal of achieving acceptable test coverage.

Revise the existing tests and determine what is worth keeping and/or what should be added to build a meaningful set of integration tests of Updater + HTTP server.