test: fix test-debug-signal-cluster.js flakyness
Checklist
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make -j4 test
(UNIX), orvcbuild 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).