Fix(tests): Instability in subscribeToKeyExpiration

FLEXISIP-341

Running the whole suite on a loop revealed the test right before it to be 1) unstable as well, and 2) the cause of the observed instability in the initial offender

Reviewer guidelines

The following requirements must be met before merging:

flexisip	done	N.A.	Tests added
	done	N.A.	Related MR pipelines succeeded
	done	N.A.	SHA1 consistency of cherry-picked commits
linphone-sdk	done	N.A.	On a tag... (Except for master, which is allowed to use yet-to-be-released versions of the SDK)
	done	N.A.	...From the corresponding branch
docs	done	N.A.	CHANGELOG up to date
	done	N.A.	wiki (public documentation) up to date