Implement `ignore` interceptor in integration test framework

[waynexia]

Description

Some cases are not expected to run due to unstable features, temporary changes or anything else. It should be supported to just ignore them.

Proposal

Implement the ignore interceptor. Proposed syntax:

-- CERESDB ignore: REASON

Follows the K-V format. Value for this interceptor is the reason why this case got ignored. It won't take effort and is a comment in fact.

Additional context

TBD: should we count/record those ignored cases and report it in the end? ref #154