Fix the notification_setting test by giving a obedient slack url

[bowenlan-amzn]

Description

Fix the notification_setting test by giving a obedient slack url
regression from opensearch-project/notifications#814

Check List

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.37%. Comparing base (be76a29) to head (cee482a).
Report is 52 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #961    +/-   ##
========================================
  Coverage   63.37%   63.37%            
========================================
  Files         341      341            
  Lines       11554    11554            
  Branches     2243     2112   -131     
========================================
  Hits         7322     7322            
  Misses       3658     3658            
  Partials      574      574            

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/index-management-dashboards-plugin/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/index-management-dashboards-plugin/backport-2.x
# Create a new branch
git switch --create backport/backport-961-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 e855cb5f9d8140c230e18bfc765096cfa0c7b8ce
# Push it to GitHub
git push --set-upstream origin backport/backport-961-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/index-management-dashboards-plugin/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-961-to-2.x.