Rails 3.0.1 で PostgreSQL 10 に繋いでテストする必要があり、 initializer を追加することになりました。

config/initializers/backport_pg_10_support_to_rails_3.rb

if Rails.env.test?
  module ActiveRecord
    class Base
      class << self
        def establish_connection(*arg)
        end
      end
    end
  end
  require 'active_record/connection_adapters/postgresql_adapter'
  module ActiveRecord
    module ConnectionAdapters
      class PostgreSQLAdapter < AbstractAdapter
        # Resets the sequence of a table's primary key to the maximum value.
        def reset_pk_sequence!(table, pk = nil, sequence = nil)
          unless pk and sequence
            default_pk, default_sequence = pk_and_sequence_for(table)
            pk ||= default_pk
            sequence ||= default_sequence
          end
          if pk
            if sequence
              quoted_sequence = quote_column_name(sequence)
              if ActiveRecord::Base.connection.select_value('SELECT version()').include?('PostgreSQL 10')
                # language=sql
                sql =<<-EOS
                  SELECT setval('#{quoted_sequence}', (SELECT GREATEST(MAX(#{quote_column_name pk})+(SELECT seqincrement FROM pg_sequence WHERE seqrelid = '#{quoted_sequence}'::regclass), (SELECT seqmin FROM pg_sequence WHERE seqrelid = '#{quoted_sequence}'::regclass)) FROM #{quote_table_name(table)}), false)
                EOS
              else
                # language=sql
                sql =<<-EOS
                  SELECT setval('#{quoted_sequence}', (SELECT GREATEST(MAX(#{quote_column_name pk})+(SELECT increment_by FROM #{quoted_sequence}), (SELECT min_value FROM #{quoted_sequence})) FROM #{quote_table_name(table)}), false)
                EOS
              end
            else
              @logger.warn "#{table} has primary key #{pk} with no default sequence" if @logger
            end
          end
        end
      end
    end
  end
end

require 'active_record/connection_adapters/postgresql_adapter' を入れて、 def reset_pk_sequence!(table, pk = nil, sequence = nil) を上書きしましたが、それだけだと establish_connection が存在しないというエラーが出るので、 require の前に establish_connection を記述しました。

reset_pk_sequence! は https://github.com/rails/rails/blob/3-0-stable/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb から基本的にコピーで、変更したのは SQL 作成部分です。