Recommend tt in remote test instructions

[Mirochill]

Summary

• note that the remote Linux test machine needs tt installed alongside Tarantool
• replace the tarantoolctl start tarantool_python_ci instruction with tt start tarantool_python_ci

Fixes #313.

Validation

• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD
• git show --check --stat HEAD
• Not run locally: tests, docs build, lint, install, import runtime, or project scripts