Fix code coverage analysis in CI

[sagebind]

Fix code coverage step in CI erroring. It seems like the primary issue here is Tarpaulin is trying to run doc tests annotated with no_run, which aren't necessarily designed to pass. Just disable doc tests in Tarpaulin to work around this since they don't contribute much to the overall code coverage anyway. I suspect a new Cargo or Rust version at some point caused this issue.

See also xd009642/tarpaulin#848.

[codecov]

Codecov Report

Merging #351 (e586d6c) into master (422d4b0) will decrease coverage by 3.75%.
The diff coverage is 28.94%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
- Coverage   75.32%   71.57%   -3.76%     
==========================================
  Files          59       60       +1     
  Lines        3218     3261      +43     
==========================================
- Hits         2424     2334      -90     
- Misses        794      927     +133     

Impacted Files	Coverage Δ
src/macros.rs	100.00% <ø> (ø)
testserver/src/server.rs	0.00% <0.00%> (ø)
src/body/mod.rs	83.17% <16.66%> (+0.93%)	⬆️
src/body/sync.rs	81.18% <25.00%> (-1.00%)	⬇️
src/error.rs	49.71% <40.00%> (-1.15%)	⬇️
src/client.rs	69.75% <100.00%> (-3.56%)	⬇️
tests/response_body.rs	100.00% <100.00%> (ø)
src/config/dns.rs	0.00% <0.00%> (-36.37%)	⬇️
src/config/mod.rs	44.02% <0.00%> (-30.10%)	⬇️
src/config/ssl.rs	18.75% <0.00%> (-16.25%)	⬇️
... and 15 more

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