Setup default loop in test utilities

[asvetlov]

The change is safe: minimal supported Python 3.5.3 always returns a running event loop if asyncio.get_event_loop() is called from a coroutine.

Also it is a step for seamless integration with pytest-asyncio: the library installs the loop internally.

[codecov-io]

Codecov Report

Merging #2804 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2804   +/-   ##
=======================================
  Coverage   97.99%   97.99%           
=======================================
  Files          39       39           
  Lines        7379     7379           
  Branches     1296     1296           
=======================================
  Hits         7231     7231           
  Misses         47       47           
  Partials      101      101

Impacted Files	Coverage Δ
aiohttp/pytest_plugin.py	98.62% <100%> (ø)	⬆️
aiohttp/test_utils.py	98.85% <100%> (ø)	⬆️

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This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new