tests: wrap passthru inside another mock

[beliaev-maksim]

I still see the issue in pipeline

https://github.com/getsentry/responses/runs/5112676859?check_suite_focus=true

I think wrap _real_send will resolve the issue. Basically, only when _real_send is called we mock it, that ensures that passthru worked.

@markstory
objections ?

[codecov]

Codecov Report

Merging #495 (6b5795a) into master (930b784) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #495   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines         2161      2162    +1     
=========================================
+ Hits          2161      2162    +1     

Impacted Files	Coverage Δ
responses/test_responses.py	100.00% <100.00%> (ø)

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