[frontend] Fix flaky custom views E2E test (#16097)

[fellowseb]

Proposed changes

Fixes flaky Custom Views E2E test by waiting for notification message displayed during duplication and import steps.
For future: avoid using a specific Campaign entity, uses first in the grid.
Also uses simpler toHaveCount() form.

Related issues

• Fixes Custom Views flaky E2E test #16097

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.71%. Comparing base (0adbde1) to head (268e50d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #16098   +/-   ##
=======================================
  Coverage   35.71%   35.71%           
=======================================
  Files        3232     3232           
  Lines      221682   221682           
  Branches    41430    41431    +1     
=======================================
  Hits        79169    79169           
  Misses     142513   142513           

Flag	Coverage Δ
opencti-client-python	46.36% <ø> (ø)
opencti-front	3.98% <ø> (ø)
opencti-graphql	72.78% <ø> (ø)

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