[sidekiq] better and isolated rails specs

[solnic]

This updates sidekiq spec setup to be inline with other projects by adding spec:isolated group and moving rails_spec.rb there because it is meant to be run in isolation where sentry-rails is required.

While working on this I also discovered that there's spec flakiness here due to the fact that redis patch is enabled by default and spec require order would make some specs randomly fail under Sidekiq <= 6.5. I fixed it by removing redis from default patches in the test config but I am wondering why this patch is enabled be default?

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.24%. Comparing base (f951c95) to head (f88a5bb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2723   +/-   ##
=======================================
  Coverage   97.24%   97.24%           
=======================================
  Files         130      130           
  Lines        5229     5229           
=======================================
  Hits         5085     5085           
  Misses        144      144           

Components	Coverage Δ
sentry-ruby	97.54% <ø> (ø)
sentry-rails	96.74% <ø> (ø)
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)

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[sl0thentr0py]

We generally default enable integrations that are widely used. Sentry SDK design is not opt-in in most cases.

What errors specifically does the patch cause?