Tweaks to make tests more robust

[philippjfr]

Attempts to improve flakyness of tests mentioned in #7118

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 28 lines in your changes missing coverage. Please review.

Project coverage is 82.22%. Comparing base (f7d7f58) to head (063a161).
Report is 2 commits behind head on main.

Files	Patch %	Lines
panel/tests/ui/widgets/test_tabulator.py	33.33%	26 Missing ⚠️
panel/tests/ui/chat/test_chat_interface_ui.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7133      +/-   ##
==========================================
- Coverage   82.26%   82.22%   -0.05%     
==========================================
  Files         327      327              
  Lines       48828    48903      +75     
==========================================
+ Hits        40168    40210      +42     
- Misses       8660     8693      +33     

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