test: retry count assertion on session invalidation

Description

Session context tests check the counter value hold by a session bean right after invalidating the Vaadin session. Since the counter is incremented in the PreDestroy hook, it may happen that when the test checks for the counter value, the callback has not yet been invoked. This change retries the counter check for at most on second, to preven ttest flakyness.

Fixes #126

Type of change

[x] Bugfix
[ ] Feature

Checklist

[x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
[x] I have added a description following the guideline.
[x] The issue is created in the corresponding repository and I have referenced it.
[ ] I have added tests to ensure my change is effective and works as intended.
[x] New and existing tests are passing locally with my change.
[ ] I have performed self-review and corrected misspellings.