test: wait logger to flush

[popomore]

Checklist

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

Affected core subsystem(s)

Description of change

fix https://travis-ci.org/eggjs/egg/jobs/245671138

[codecov]

Codecov Report

Merging #1090 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1090   +/-   ##
======================================
  Coverage    99.7%   99.7%           
======================================
  Files          29      29           
  Lines         676     676           
======================================
  Hits          674     674           
  Misses          2       2

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