Testing specific sequences the client should be able to handle

[sechkova]

Description of issue or feature request:

Consider different types of updates sequences that may exist in a repository and which the updater should be able to handle.
Analyze their relevance and the need of a dedicated test case. For example:

• Client that has been offline for N expirations of root metadata
• Air-gapped client outside of timestamp expiration window
• Some of the previously reported security issues