test: return with promise

[ahungrynoob]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

之前ts-cluster的测试用例不稳定： #138
采用这种写法就没问题了。
本地ci测试结果：https://github.com/ahungrynoob/test-egg-bin/pull/1

Description of change

[codecov]

Codecov Report

Merging #143 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #143   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         229    229           
=====================================
  Hits          229    229

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