test: add unit tests for dragonfly-backend::http

[Lzzzzzt]

Add https related unit tests for dragonfly-backend::http.

Description

This PR is done by finishing the following tasks:

• Add test for get/head with self-signed cert.
• Add test for get/head with wrong cert.
• Add test for get/head with no verifier.

Related Issue

#399

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.32%. Comparing base (3973842) to head (6495732).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #770      +/-   ##
==========================================
+ Coverage   27.48%   29.32%   +1.84%     
==========================================
  Files          55       55              
  Lines        7724     7883     +159     
==========================================
+ Hits         2123     2312     +189     
+ Misses       5601     5571      -30     

Files with missing lines	Coverage Δ
dragonfly-client-backend/src/http.rs	97.09% <100.00%> (+5.70%)	⬆️
dragonfly-client-core/src/error/mod.rs	66.66% <ø> (ø)

... and 1 file with indirect coverage changes

[gaius-qi]

LGTM