test: fix test-debug-signal-cluster.js flakyness

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

Tests.

Description of change

Do not assume any order and buffering/atomicity of output from child processes' debugger agents.

Fixes #3796 (closed).