Add test for mocking library

[delvedor]

As titled, this should help to catch errors before release.

Wait for elastic/elasticsearch-js-mock#20.

[github-actions]

The backport to 7.x failed:

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

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-7.x 7.x
# Navigate to the new working tree
cd .worktrees/backport-7.x
# Create a new branch
git switch --create backport-1545-to-7.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 b67d42cb5ff7a39a1836d176266ac32af9e72f07
# Push it to GitHub
git push --set-upstream origin backport-1545-to-7.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.x

Then, create a pull request where the base branch is 7.x and the compare/head branch is backport-1545-to-7.x.