Remove faraday from tests

[st0012]

This is the first step of removing faraday from dependencies: instead of using its stubbing helpers, we can build a set of slightly lower-level helpers to replace them. I've already done a similar thing in net/http_spec so we can reuse most of the concept here.

[codecov-commenter]

Codecov Report

Merging #1622 (681e263) into master (23c66f5) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 681e263 differs from pull request most recent head 3bd2296. Consider uploading reports for the commit 3bd2296 to get more accurate results

@@            Coverage Diff             @@
##           master    #1622      +/-   ##
==========================================
- Coverage   98.50%   98.50%   -0.01%     
==========================================
  Files         132      133       +1     
  Lines        7341     7336       -5     
==========================================
- Hits         7231     7226       -5     
  Misses        110      110              

Impacted Files	Coverage Δ
sentry-ruby/spec/contexts/with_request_mock.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/net/http_spec.rb	100.00% <100.00%> (ø)
...try/transport/http_transport_rate_limiting_spec.rb	99.11% <100.00%> (-0.04%)	⬇️
...-ruby/spec/sentry/transport/http_transport_spec.rb	100.00% <100.00%> (ø)

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