Added documentation about testing with the explicit loop

[argaen]

What do these changes do?

Updates on documentation about how to test by passing the loop explicitly and why you should do that.

Related issue number

#1069

[codecov-io]

Current coverage is 97.71% (diff: 100%)

Merging #1097 into master will not change coverage

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[asvetlov]

Thanks!
Feel free for adding very explained texts about aiohttp problems and best usage practices like this one.

[argaen]

Sure! :)

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