[Enhancement] Replace JUnit assertEquals() with Hamcrest matchers assertThat()

[prabhask5]

Description

As per this issue: #1832, changed all occurrences of Assert.assertEquals to assertThat, additionally changed imports accordingly.

• Category (Enhancement, New feature, Bug fix, Test fix, Refactoring, Maintenance, Documentation)
  Enhancement

• Why these changes are required?
  Refer to: [Test Refactoring] Replace Assert.assertEquals() with assertThat() #1832

• What is the old behavior before changes and new behavior after changes?
  Nothing ideally.

Issues Resolved

#3680
#1832

Is this a backport? If so, please add backport PR # and/or commits #
No

Testing

N/A, changed tests

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[prabhask5]

@scrawfor99 @cwperks @DarshitChanpura Ready for review! Had some commit issues in the previous PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.00%. Comparing base (9d32a8c) to head (7b95b7f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4544      +/-   ##
==========================================
+ Coverage   64.99%   65.00%   +0.01%     
==========================================
  Files         314      314              
  Lines       22111    22111              
  Branches     3566     3566              
==========================================
+ Hits        14370    14374       +4     
+ Misses       5955     5951       -4     
  Partials     1786     1786              

see 2 files with indirect coverage changes

[DarshitChanpura]

nicely done @prabhask5

[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/security/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/security/backport-2.x
# Create a new branch
git switch --create backport/backport-4544-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 0b2109a5d50dc5ae16a1ccd641645433b9da8a7c
# Push it to GitHub
git push --set-upstream origin backport/backport-4544-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/security/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-4544-to-2.x.

[DarshitChanpura]

@prabhask5 mind creating a manual backport to 2.x?