Improve `BeforeAllAdapter` support

Usage of `connection_handlers` is not always correct
as we might end-up with misaligned transactions.

The usage of `AR::Base.establish_connection` is bad design
as it break `before_all_adapter` and is always a misaligned transaction.

The cases where it will break:
- `.establish_connection`
- `.pool.disconnect!`
- `.release_connection`

spec/support/before_all_adapter.rb

 # frozen_string_literal: true
 
 class BeforeAllAdapter # rubocop:disable Gitlab/NamespacedClass
-  def self.all_connection_pools
-    ::ActiveRecord::Base.connection_handler.all_connection_pools
+  def self.all_connection_classes
+    @all_connection_classes ||= [ActiveRecord::Base] + ActiveRecord::Base.descendants.select(&:connection_class?) # rubocop: disable Database/MultipleDatabases
   end
 
   def self.begin_transaction
-    self.all_connection_pools.each do |connection_pool|
-      connection_pool.connection.begin_transaction(joinable: false)
+    self.all_connection_classes.each do |connection_class|
+      connection_class.connection.begin_transaction(joinable: false)
     end
   end
 
   def self.rollback_transaction
-    self.all_connection_pools.each do |connection_pool|
-      if connection_pool.connection.open_transactions.zero?
+    self.all_connection_classes.each do |connection_class|
+      if connection_class.connection.open_transactions.zero?
         warn "!!! before_all transaction has been already rollbacked and " \
               "could work incorrectly"
         next
       end
 
-      connection_pool.connection.rollback_transaction
+      connection_class.connection.rollback_transaction
     end
   end
 end