introduce region error handler unit test

[iosmanthus]

Signed-off-by: iosmanthus myosmanthustree@gmail.com<!-- Thank you for contributing to TiKV Java Client!

PR Title Format: "[close/to/fix #issue_number] summary" -->

What problem does this PR solve?

Issue Number: close #issue_number

Problem Description: TBD

What is changed and how does it work?

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has methods of interface change
• Has persistent data change
• No code

Check List for Tests

This PR has been tested by at least one of the following methods:

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Possible performance regression, WHY: TBD
• Increased code complexity, WHY: TBD
• Breaking backward compatibility, WHY: TBD
• NO side effects

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to be included in the release note
• NO related changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release-3.3.1.14@903fbde). Learn more about missing BASE report.

Additional details and impacted files

@@                 Coverage Diff                 @@
##             release-3.3.1.14     #771   +/-   ##
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  Coverage                    ?   30.85%           
  Complexity                  ?     1173           
===================================================
  Files                       ?      266           
  Lines                       ?    14168           
  Branches                    ?     1592           
===================================================
  Hits                        ?     4372           
  Misses                      ?     9433           
  Partials                    ?      363           

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