重构task pool相关测试用例

[longyue0521]

1. 重构TestOnDemandBlockTaskPool_States,只关注是否能读取数据,读取的数据是否为零值,返回的chan是否被关闭等不再校验数据内容.
2. 完善Taskpool有限状态机
3. 添加t.Parallel() 尽可能利用Go测试框架的并发特性加速测试用例运行
4. TestOnDemandBlockTaskPool_In_Closing_State/Shutdown

[codecov]

Codecov Report

Merging #178 (eaae95c) into dev (155780c) will decrease coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #178      +/-   ##
==========================================
- Coverage   96.14%   95.89%   -0.26%     
==========================================
  Files          43       43              
  Lines        2362     2362              
==========================================
- Hits         2271     2265       -6     
- Misses         73       77       +4     
- Partials       18       20       +2     

see 2 files with indirect coverage changes

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[longyue0521]

@flycash 有空看一下