Add redis-tools as a test-only docker dependency

[jayz22]

Description

redis-cli is needed for the container to communicate to the Redis host.
The new parallel catchup mission stellar/supercluster#178 requires access to Redis task queue.
This is only added as a test dependency i.e. package version with *~buildtests, and does not affect production stellar-core packages.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document