test: wait on the EPS cancellation the T3550 expiry test asserts

[gruyaume]

Description

A test could report a false failure in CI depending on timing, so it now waits for the behavior it checks.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules