testUpdateExtensionSubscription is often failing on Pharo12

[jecisc]

This test was previously failing from time to time on Pharo's CI but recently it is almost always failing on Pharo 12 CI but it's almost always on OSX.

This is probably related to something random but maybe it can be used to find the origin? (For example doing a PR with logs in the test to find the origin?)

Example of stack

osx-64 / Tests-osx-64 / testUpdateExtensionSubscription – MacOSX64.NewTools.Debugger.Tests.StDebuggerTest8s
Error
Got 1 instead of 5.
Stacktrace
TestFailure
Got 1 instead of 5.
StDebuggerTest(TestAsserter)>>assert:description:resumable:
StDebuggerTest(TestAsserter)>>assert:description:
StDebuggerTest(TestAsserter)>>assert:equals:
StDebuggerTest>>testUpdateExtensionSubscription
StDebuggerTest(TestCase)>>performTest

https://ci.inria.fr/pharo-ci-jenkins2/blue/organizations/jenkins/Test%20pending%20pull%20request%20and%20branch%20Pipeline/detail/PR-15488/1/tests

Can maybe interest @StevenCostiou or @adri09070