test: improve unit test coverage of seata-tm moudle

[YvCeung]

• I have registered the PR changes.

Ⅰ. Describe what this PR did

Add unit test cases to complete the unit testing objectives for the seata-tm module.
增加单元测试用例，完成 seata-tm模块的单元测试目标

Ⅱ. Does this pull request fix one issue?

fix #6837

Ⅲ. Why don't you add test cases (unit test/integration test)?

Not applicable.
不适用

Ⅳ. Describe how to verify it

Run mvn clean test

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.61%. Comparing base (289f07d) to head (510f307).
Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7018      +/-   ##
============================================
+ Coverage     52.52%   52.61%   +0.08%     
- Complexity     6577     6593      +16     
============================================
  Files          1126     1126              
  Lines         40007    40007              
  Branches       4696     4694       -2     
============================================
+ Hits          21015    21051      +36     
+ Misses        16984    16948      -36     
  Partials       2008     2008              

see 8 files with indirect coverage changes

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[funky-eyes]

LGTM