Disable Sidekiq worker open transaction check in JS specs

What does this MR do and why?

When transactional tests are enabled, Rails ensures that all threads use the same connection for accessing the database. However this may cause a false positive with the Sidekiq worker open transaction check because:

1. A browser may have multiple requests in-flight.
2. A transaction may start in the middle of another request. For example, an update to a merge request column initiates a transaction via SAVEPOINT.

To avoid false positives, disable this check for JavaScript feature specs run. The alternative is to use the deletion strategy, but that would significantly slow tests.

Relates to #478601 (closed)

MR acceptance checklist

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How to set up and validate locally

Repeatedly run:

bundle exec rspec ./spec/features/merge_request/user_squashes_merge_request_spec.rb

config/initializers/forbid_sidekiq_in_transactions.rb

@@ -13,6 +13,16 @@ def self.skipping_transaction_check(&block)
     end
 
     def self.skip_transaction_check
+      # When transactional tests are in use, Rails calls
+      # ConnectionPool#lock_ethread= to ensure all application threads
+      # get the same connection so they can all see the data in the
+      # uncommited transaction. If Puma is in use, check the state of
+      # the lock thread.
+      if ::Rails.env.test?
+        lock_thread = ::ApplicationRecord.connection_pool.instance_variable_get(:@lock_thread)
+        return true if lock_thread && lock_thread[:sidekiq_worker_skip_transaction_check]
+      end
+
       Thread.current[:sidekiq_worker_skip_transaction_check]
     end