refactor passthru test functions

[beliaev-maksim]

• move all passthru functions into one class
• rewritten test for test_passthru_does_not_persist_across_tests. Instead of overriding _real_send we rely on httpserver as other tests do

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c25a1c8) 100.00% compared to head (f55d15b) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #646 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 9 9 Lines 2872 2873 +1 ========================================= + Hits 2872 2873 +1 ``` | [Impacted Files](https://app.codecov.io/gh/getsentry/responses/pull/646?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getsentry) | Coverage Δ | | |---|---|---| | [responses/tests/test\_responses.py](https://app.codecov.io/gh/getsentry/responses/pull/646?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getsentry#diff-cmVzcG9uc2VzL3Rlc3RzL3Rlc3RfcmVzcG9uc2VzLnB5) | `100.00% <100.00%> (ø)` | |

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