Apply OpenSearchIntegTestCase::featureFlagSettings by default

[andrross]

This commit applies any feature flags specified by a child class override of the featureFlagSettings() method by default. The result is that test implementations do not have to manually specify the featureFlagSettings when creating test nodes.

@dreamer-89 @mch2 Does this make sense?

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[dreamer-89]

LGTM! Thanks @andrross for this change. Yes, this change is useful.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/9610/
• CommitID: 5a6e2bf
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[andrross]

Build succeeded, but the scripted failed to get the result from the Jenkins server:

Complete the run, checking results now......
parse error: Invalid numeric literal at line 1, column 7

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.cluster.routing.allocation.decider.DiskThresholdDeciderIT.testIndexCreateBlockWhenAllNodesExceededHighWatermark

• URL: https://build.ci.opensearch.org/job/gradle-check/9613/
• CommitID: 5a6e2bf
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[mch2]

Yep this is a great simplification, thanks @andrross.

[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:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-5940-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7a511dd9ac7989d27d0b4aa7a0b0bf43fb36a91a
# Push it to GitHub
git push --set-upstream origin backport/backport-5940-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-5940-to-2.x.