Resolve concurrency with test stub to resolve #49745.

[feherzsolt]

Eliminate concurrentcy with test stub

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Summary of the changes (Less than 80 chars)

Description

TestPublisher was using a List which was modified from multiple concurrent Task which might have led to test flakiness. I have replaced the collection with a thread safe one to try resolving this flakiness.

(Hopefully)
Fixes #49745