Refactor test server to support writing raw response data

[sagebind]

This is needed to test HTTP/0.9 responses, and is also generally useful. The mock! macro also now allows easily defining multiple responses based on the request.

[codecov]

Codecov Report

Merging #366 (2bf06f8) into master (053a927) will decrease coverage by 0.00%.
The diff coverage is 97.29%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
- Coverage   79.38%   79.37%   -0.01%     
==========================================
  Files          51       51              
  Lines        3061     3060       -1     
==========================================
- Hits         2430     2429       -1     
  Misses        631      631              

Impacted Files	Coverage Δ
tests/auth.rs	100.00% <ø> (ø)
tests/proxy.rs	71.87% <75.00%> (ø)
tests/headers.rs	97.47% <100.00%> (-0.03%)	⬇️
tests/methods.rs	100.00% <100.00%> (ø)
tests/metrics.rs	100.00% <100.00%> (ø)
tests/net.rs	92.42% <100.00%> (ø)
tests/redirects.rs	100.00% <100.00%> (ø)
tests/request_body.rs	100.00% <100.00%> (ø)
tests/response_body.rs	100.00% <100.00%> (ø)
tests/status.rs	100.00% <100.00%> (ø)
... and 1 more

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