Test for commands in backup.actor.cpp

Backup.actor.cpp is used for many purposes: backupAgent, fdbrestore, fastrestore_tool.

Similar to the lack of good tests for `fdbcli` (e.g., https://github.com/apple/foundationdb/issues/3565), we don't have a good testing story for backup.actor.cpp derived tools. As a result, the command for those tools can be broken silently until someone uses it.

Changing to use special key space framework to replace `fdbcli` can solve `fdbcli` issue, but not backup.actor.cpp issue.

Ideally, fuzz testing for the tool commands will be best.

Start by reading backup.actor.cpp and tracing the command paths used by backupAgent, fdbrestore, and fastrestore_tool. Compare their command coverage with the fdbcli testing gap; done means command behavior is covered by a defined test or fuzz-testing approach so regressions are detected.

Written by the indexing model from the issue text.