fix test_multiple_drains (was not covering what it should)

[arthurdarcet]

@asvetlov : sorry, should not have pushed my last PR #2126 so soon. I was still checking my test were covering everything, and the test_multiple_drains was not doing anything useful. This commit fixes it (and should improve the coverage a very tiny bit)

[codecov-io]

Codecov Report

Merging #2553 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #2553      +/-   ##
=========================================
+ Coverage   97.29%   97.3%   +0.01%     
=========================================
  Files          40      40              
  Lines        8134    8134              
  Branches     1432    1432              
=========================================
+ Hits         7914    7915       +1     
  Misses         89      89              
+ Partials      131     130       -1

Impacted Files	Coverage Δ
aiohttp/http_writer.py	98.35% <0%> (+0.54%)	⬆️

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[asvetlov]

No problem.
Is the PR ready for merging?

[arthurdarcet]

yes, all good

[asvetlov]

Cool, thanks!

[lock]

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[new issue]: https://github.com/aio-libs/aiohttp/issues/new