Add test for 0 timeout

[Dreamsorcerer]

Regression test ported from #5529, plus minor refactoring of ceil_timeout().

[codecov]

Codecov Report

Merging #5533 (b7bd646) into master (71f574f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5533   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files          41       41           
  Lines        8854     8854           
  Branches     1424     1424           
=======================================
  Hits         8604     8604           
  Misses        133      133           
  Partials      117      117           

Flag	Coverage Δ
unit	97.06% <100.00%> (ø)

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Impacted Files	Coverage Δ
aiohttp/helpers.py	96.77% <100.00%> (ø)

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