Add missing reply schema and coverage tests (#12079)

The change in #12018 break the CI (fixed by #12083).
There are quite a few sentinel commands that are missing both test coverage and also schema.

PR added reply-schema to the following commands:
- sentinel debug
- sentinel info-cache
- sentinel pendding-scripts
- sentinel reset
- sentinel simulate-failure

Added some very basic tests for other sentinel commands, just so that they have some coverage.
- sentinel help
- sentinel masters
- sentinel myid
- sentinel sentinels
- sentinel slaves

These tests should be improved / replaced in a followup PR.