Add an xfailing test expecting HTTP500 for invalid handler return values

[Fogapod]

This change documents a contract of disallowing non-response
return values in request handlers that currently has a bug and
is therefore marked as xfail.

Issue: #4572

Created test as @webknjaz suggested.

[Fogapod]

Should test for returning HTTPException also be included? I'm not sure how it should be checked because it doesn't work at all currently.

[codecov-io]

Codecov Report

Merging #4574 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4574      +/-   ##
==========================================
+ Coverage   97.53%   97.57%   +0.04%     
==========================================
  Files          43       43              
  Lines        8907     8907              
  Branches     1404     1404              
==========================================
+ Hits         8687     8691       +4     
+ Misses         98       96       -2     
+ Partials      122      120       -2     

Impacted Files	Coverage Δ
aiohttp/web_runner.py	97.78% <0.00%> (ø)	⬆️
aiohttp/tcp_helpers.py	93.33% <0.00%> (ø)	⬆️
aiohttp/pytest_plugin.py	97.43% <0.00%> (+1.92%)	⬆️
aiohttp/connector.py	96.28% <0.00%> (ø)	⬆️
aiohttp/test_utils.py	99.68% <0.00%> (ø)	⬆️
aiohttp/web_protocol.py	92.69% <0.00%> (+0.29%)	⬆️

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[webknjaz]

I'd also check if the text includes something like "Internal Server Error".

[webknjaz]

Should test for returning HTTPException also be included?

Feel free to add a separate PR.

v3.6 deprecates returning exceptions but it still works (see the red warning box): https://docs.aiohttp.org/en/stable/web_quickstart.html#exceptions

master/v4.0 doesn't support this anymore and doesn't mention returning exceptions: https://docs.aiohttp.org/en/latest/web_exceptions.html

Based on this, when returning errors under 4.0/master, check for HTTP 500 Internal Server Error.