test readability: refactored TLS tests

[fredbi]

This PR refactors TLS tests:

• factorized the clutter of dealing with TLS fixtures (certs, keys, etc)
• separated ancillary tasks such as initializing a TLS http server and client
• removed assertions on the deprecated CertPool. The only thing we can do with a CertPool is to verify a certificate against the pool

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7c6b268) 80.82% compared to head (4f2e629) 80.82%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #276   +/-   ##
=======================================
  Coverage   80.82%   80.82%           
=======================================
  Files          44       44           
  Lines        3364     3364           
=======================================
  Hits         2719     2719           
  Misses        535      535           
  Partials      110      110           

Flag	Coverage Δ
oldstable	80.82% <ø> (ø)
stable	80.82% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.