[Backport 2.x] Document testing of discovery-ec2 and repository-s3.

[opensearch-trigger-bot]

Backport 4228075 from #7868.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      2 org.opensearch.search.SearchWeightedRoutingIT.testStrictWeightedRoutingWithCustomString
      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testPressureServiceStats
      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testDropPrimaryDuringReplication
      1 org.opensearch.remotestore.RemoteStoreRefreshListenerIT.testRemoteRefreshRetryOnFailure
      1 org.opensearch.indices.replication.SegmentReplicationTargetServiceTests.testStartReplicationListenerSuccess

• URL: https://build.ci.opensearch.org/job/gradle-check/17802/
• CommitID: 3dc0017
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Merging #8116 (3dc0017) into 2.x (59a62e7) will increase coverage by 0.15%.
The diff coverage is 78.57%.

@@             Coverage Diff              @@
##                2.x    #8116      +/-   ##
============================================
+ Coverage     70.63%   70.79%   +0.15%     
- Complexity    56771    56873     +102     
============================================
  Files          4703     4703              
  Lines        269046   269056      +10     
  Branches      39768    39770       +2     
============================================
+ Hits         190048   190467     +419     
+ Misses        62694    62234     -460     
- Partials      16304    16355      +51     

Impacted Files	Coverage Δ
.../org/opensearch/discovery/ec2/Ec2NameResolver.java	89.47% <0.00%> (ø)
.../org/opensearch/common/settings/SecureSetting.java	74.62% <84.61%> (+9.71%)	⬆️

... and 486 files with indirect coverage changes