add sessionManager unit test

[xm-king]

Ⅰ. Describe what this PR did

add SessionManager Unit Test

Ⅱ. Does this pull request fix one issue?

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

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #222 into develop will increase coverage by 3.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop    #222      +/-   ##
==========================================
+ Coverage     4.62%   8.21%   +3.58%     
==========================================
  Files          184     184              
  Lines         6770    6770              
  Branches       931     931              
==========================================
+ Hits           313     556     +243     
+ Misses        6439    6163     -276     
- Partials        18      51      +33

Impacted Files	Coverage Δ
.../java/com/alibaba/fescar/server/UUIDGenerator.java	17.39% <0%> (+17.39%)	⬆️
...scar/server/store/FileTransactionStoreManager.java	25.53% <0%> (+25.53%)	⬆️
...m/alibaba/fescar/server/session/GlobalSession.java	27.97% <0%> (+27.97%)	⬆️
...aba/fescar/server/store/TransactionWriteStore.java	36.11% <0%> (+36.11%)	⬆️
...m/alibaba/fescar/server/session/BranchSession.java	36.76% <0%> (+36.76%)	⬆️
...a/fescar/server/store/TransactionStoreManager.java	42.3% <0%> (+42.3%)	⬆️
...a/fescar/server/session/DefaultSessionManager.java	57.14% <0%> (+57.14%)	⬆️
...ba/fescar/server/store/TransactionWriteFuture.java	58.06% <0%> (+58.06%)	⬆️
...libaba/fescar/server/session/SessionCondition.java	60% <0%> (+60%)	⬆️
.../fescar/server/session/AbstractSessionManager.java	85.18% <0%> (+85.18%)	⬆️
... and 1 more

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[slievrly]

@xm-king good job.

[slievrly]

approval