fix(local): wait for stopped child processes

[andrussal]

This PR makes local test node cleanup wait for child processes to actually exit after sending the kill signal.

Previously local nodes were only asked to stop, but the framework did not wait for the process to be reaped before clearing node state. That can leave short-lived stale processes around and keep ports/resources busy after tests finish or fail.

[hansieodendaal]

LGTM

Nit: We may want to spin up some concurrent futures for this:

for node in &mut state.nodes {
    node.stop_blocking();
}