ngtests: add tests toggling consistent snapshot

[sechkova]

Description of issue or feature request:
Currently different configurations of consistent_snapshot, prefix_targets_with_hash are tested in test_updater_ng.py which uses pre-generated test files.

After adding support for non-consistent snapshot #1637, analyze what would be meaningful consistent snapshot tests that preferably don't duplicate the tests code for each config and implement them using RepositorySimulator.

[jku]

After adding support for non-consistent snapshot #1637, analyze what would be meaningful consistent snapshot tests that preferably don't duplicate the tests code

Sounds good.
I think it's reasonable to concentrate testing on

• consistent_snapshot with consistent targets
• consistent_snapshot without consistent targets (the special case for warehouse)

and leave the non-consistent_snapshot case with just the minimum tests to verify it works: everyone should be using consistent_snapshot, we only support the other case because the spec still allows it.