Use adminClient when searching system index in integ tests

[cwperks]

Description

This PR fixes test failures when running with Security. There are instances in integ tests where the integ test will directly search a system index and should be done with the adminClient.

The reason the tests passed in 2.17 and are failing now is due to a bugfix going into 2.18: opensearch-project/security#4774

In 2.17, security started protecting system indices registered with SystemIndexPlugin.getSystemIndexDescriptors() by default. Admin operations like delete index or write to index are explicitly denied for all users except a user connecting with the admin certificate. Search operations will give back a 200 response, but it will return empty results. In 2.17 there was an issue with the indices registered in core that allowed them to be directly searched by an admin user that has been fixed in 2.18.

Related Issues

Resolves #1269

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

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/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/index-management/backport-2.x
# Create a new branch
git switch --create backport/backport-1286-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ca2ada20d59be44d0cd10c38658eb9eb20c1d64c
# Push it to GitHub
git push --set-upstream origin backport/backport-1286-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/index-management/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-1286-to-2.x.

[opensearch-trigger-bot]

The backport to 2.18 failed:

The process '/usr/bin/git' failed with exit code 128

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/backport-2.18 2.18
# Navigate to the new working tree
pushd ../.worktrees/index-management/backport-2.18
# Create a new branch
git switch --create backport/backport-1286-to-2.18
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ca2ada20d59be44d0cd10c38658eb9eb20c1d64c
# Push it to GitHub
git push --set-upstream origin backport/backport-1286-to-2.18
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/index-management/backport-2.18

Then, create a pull request where the base branch is 2.18 and the compare/head branch is backport/backport-1286-to-2.18.