test: fix lighting unit test TestBatchSplitRegionByRanges and a panic issue (#1086)

[ti-chi-bot]

This is an automated cherry-pick of #1086

What problem does this PR solve?

• Make the function call count atomic since the BatchSplitRegions method is now called in multiple routines.
• Do not try to flush and unsafe ingest closed engine.

What is changed and how it works?

Check List

Tests

• Unit test

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release Note

• No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Details

Reviewer can indicate their review by writing /lgtm in a comment.
Reviewer can cancel approval by writing /lgtm cancel in a comment.

[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved (/lgtm + /merge) by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[lonng]

@glorv Please resolve the conflicts.