Add ivars that are present in ActiveRecord::TestFixtures to ManagesTransactions

These ivars are normally initialized by `setup_fixtures`, which isn't actually mixed into
cypress-rails, because we're piggy-backing on AR's fixture transaction management
feature, not actually using fixtures. As a result, this was probably never actually working
correctly for applications that spun up >1 connection in their test runs.

However, because we bind the connection pool call to self, this should at least get that
much working, though without a reproducible test case my confidence that this ever
worked as imagined or will start working now is very low. Nevertheless, should clear
the error being experienced without impacting anyone for whom this actually is
working as intended somehow.

Verified these ivars are present thru Rails 6.1.4

Fixes #88
Fixes #89

Author: searls

lib/cypress-rails/manages_transactions.rb

@@ -68,6 +68,9 @@ def gather_connections
     # need to share a connection pool so that the reading connection
     # can see data in the open transaction on the writing connection.
     def setup_shared_connection_pool
+      @legacy_saved_pool_configs ||= Hash.new { |hash, key| hash[key] = {} }
+      @saved_pool_configs ||= Hash.new { |hash, key| hash[key] = {} }
+
       ActiveRecord::TestFixtures.instance_method(:setup_shared_connection_pool).bind_call(self)
     end
   end