Add the missing TestClient.scheme property

[emilmelnikov]

What do these changes do?

Add the missing TestClient.scheme property that is mentioned in the testing documentation but has not been implemented.

Are there changes in behavior for the user?

Users can access the scheme property of the test client:

async def test_spam(aiohttp_client):
    client = aiohttp_client(my_app_factory())
    assert client.scheme == 'http'

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov-io]

Codecov Report

Merging #3721 into master will decrease coverage by 3.48%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #3721      +/-   ##
=========================================
- Coverage   97.88%   94.4%   -3.49%     
=========================================
  Files          43      43              
  Lines        8559    8561       +2     
  Branches     1377    1376       -1     
=========================================
- Hits         8378    8082     -296     
- Misses         75     360     +285     
- Partials      106     119      +13

Impacted Files	Coverage Δ
aiohttp/test_utils.py	97.76% <100%> (-1.6%)	⬇️
aiohttp/worker.py	6.55% <0%> (-90.99%)	⬇️
aiohttp/resolver.py	46.55% <0%> (-53.45%)	⬇️
aiohttp/web_fileresponse.py	66.66% <0%> (-29.89%)	⬇️
aiohttp/tcp_helpers.py	63.33% <0%> (-26.67%)	⬇️
aiohttp/typedefs.py	91.3% <0%> (-8.7%)	⬇️
aiohttp/pytest_plugin.py	89.79% <0%> (-7.49%)	⬇️
aiohttp/locks.py	92.85% <0%> (-7.15%)	⬇️
aiohttp/web_runner.py	91.7% <0%> (-6.22%)	⬇️
aiohttp/__init__.py	94.44% <0%> (-5.56%)	⬇️
... and 12 more

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[asvetlov]

Thanks!