Refactor Project integration settings spec to use shared examples

What does this MR do and why?

Continuation of !80215 (closed) due to #351594 (closed)

This MR

• refactors the integration_settings_form spec to use Shared examples
• adds a spec for group level integration settings form
• adds a spec for instance level integration settings form

Describe in detail what your merge request does and why.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. bundle exec rspec spec/features/projects/integrations/project_integrations_spec.rb
2. bundle exec rspec spec/features/groups/integrations/group_integrations_spec.rb
3. bundle exec rspec spec/features/admin/integrations/instance_integrations_spec.rb

MR acceptance checklist

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