[rails] fix / simplify and improve test rails app setup

[solnic]

This fixes and simplifies test rails app setup for our sentry-rails test suite. Previous solution suffered from too much duplication and global state pollution. I tried to stick to a regular Rails setup as much as possible here.

In summary:

• Use same models/controllers/views for all rails test app versions because they are identical except PostsController#attach which needed special handling of service_name for Rails 6.1+
• Use regular db/schema.rb file and load it just once in before(:suite)
• Derive version-specific Sentry::TestRailsApp from a base Sentry:Rails:::Test::Application so that we can just leverage regular inheritance for version-specific tweaks
• Do not allow nested make_basic_app calls - this causes issues as more global bloat is created (hence some specs were tweaked to not do this)
• Move make_basic_app to a dedicated Sentry::Rails::TestHelper that gets included in all spec examples

This makes the spec suite more stable and faster:

• On 6.1 ~40s down to ~16s
• On 7.1 ~1.5m down to ~1m
• On 8.0 ~4.5m down to ~3.25m
• On 8.1 ~4m down to ~3m

Notice that the suite gets slower and slower with each Rails version. This is because Rails accumulates more global state and there are still some leakages. We can keep improving it every time there's a chance. I'm gonna call it a day for now as it's already a huge improvement 😄

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.41%. Comparing base (997ef4e) to head (4283b8c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2762      +/-   ##
==========================================
- Coverage   97.25%   90.41%   -6.84%     
==========================================
  Files         130      131       +1     
  Lines        5240     5250      +10     
==========================================
- Hits         5096     4747     -349     
- Misses        144      503     +359     

Components	Coverage Δ
sentry-ruby	97.60% <ø> (+0.05%)	⬆️
sentry-rails	55.84% <ø> (-40.94%)	⬇️
sentry-sidekiq	95.05% <ø> (ø)
sentry-resque	94.44% <ø> (ø)
sentry-delayed_job	94.68% <ø> (ø)
sentry-opentelemetry	99.31% <ø> (ø)
see 20 files with indirect coverage changes

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[cursor]

Bug: Pathname to Require: Potential String Conversion Pitfall

The require statement at line 76 is passed a Pathname object from Test::Application.schema_file. While Pathname objects often implicitly convert to a string path, require expects a string, which could lead to unexpected behavior or errors in certain Ruby versions or environments.