Remove forked from test_transport and generalize capturing_server to be module level

[sl0thentr0py]

CapturingServer now has pytest.request level isolation and can be used just once per the module instead of on every test fork/run. As a result, test_transport now takes 14 seconds instead of almost 3 minutes.

BEFORE

 235 passed, 384 skipped in 173.78s (0:02:53)

AFTER

 235 passed, 384 skipped in 13.79s

part of #4538

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.34%. Comparing base (1cba56a) to head (c9b3a79).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4577      +/-   ##
==========================================
- Coverage   80.71%   80.34%   -0.38%     
==========================================
  Files         156      156              
  Lines       16483    16483              
  Branches     2801     2801              
==========================================
- Hits        13304    13243      -61     
- Misses       2297     2362      +65     
+ Partials      882      878       -4     

see 8 files with indirect coverage changes