371d7f120 added a change that configures the tcp (plaintext) port
alongside the tls port, this causes the INFO command for tcp_port
to return that instead of the tls port when running in tls, and that broke
the sentinel tests that query it.
the fix is to add a method that gets the right port from CONFIG instead
of relying on the tcp_port info field.
The test now runs in a self-contained directory.
The general abstractions to run the tests in an environment where
mutliple instances are executed at the same time was extrapolated into
instances.tcl, that will be reused to test Redis Cluster.