Add isolated tests to sentry-rails

[st0012]

Some initialization logics are hard to test with the normal RSpec setup. Isolate each cases into their own files will make it easier (but also slower).

[codecov-commenter]

Codecov Report

Merging #1497 (bbec128) into master (083a0d6) will increase coverage by 0.56%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1497      +/-   ##
==========================================
+ Coverage   98.21%   98.77%   +0.56%     
==========================================
  Files         218      123      -95     
  Lines       10553     6716    -3837     
==========================================
- Hits        10365     6634    -3731     
+ Misses        188       82     -106     

Impacted Files	Coverage Δ
sentry-raven/spec/raven/client_state_spec.rb
sentry-raven/lib/raven/backtrace.rb
sentry-raven/lib/raven/interface.rb
sentry-raven/spec/raven/integrations/rails_spec.rb
sentry-raven/lib/raven/cli.rb
...ations/rails/overrides/debug_exceptions_catcher.rb
sentry-raven/lib/raven/processor/cookies.rb
...entry-raven/lib/raven/breadcrumbs/sentry_logger.rb
...lib/raven/integrations/rails/controller_methods.rb
...ntry-raven/lib/raven/core_ext/object/duplicable.rb
... and 85 more

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