Eagerly materialize the fixtures transaction

The transaction used to restore fixtures is an implementation detail
that should be abstracted away. Idealy a test should behave the same
wether or not transactional fixtures are enabled.

However since transactions have been made lazy, the fixture
transaction started leaking into tests case. e.g. consider the
following (oversimplified) test:

```ruby
class SQLSubscriber
  attr_accessor :sql

  def initialize
    @sql = []
  end

  def call(*, event)
    sql << event[:sql]
  end
end

subscriber = SQLSubscriber.new
ActiveSupport::Notifications.subscribe("sql.active_record", subscriber)

User.connection.execute('SELECT 1', 'Generic name')
assert_equal ['SELECT 1'], subscriber.sql
```

On Rails 6 it starts to break because the `sql` array will be `['BEGIN', 'SELECT 1']`.

Several things are wrong here:

  - That transaction is not generated by the tested code, so it shouldn't be visible.
  - The transaction is not even closed yet, which again doesn't reflect the reality.

activerecord/lib/active_record/connection_adapters/abstract/transaction.rb

@@ -205,9 +205,12 @@ module ActiveRecord
                                        run_commit_callbacks: run_commit_callbacks)
             end
 
-          transaction.materialize! unless @connection.supports_lazy_transactions? && lazy_transactions_enabled?
+          if @connection.supports_lazy_transactions? && lazy_transactions_enabled? && options[:_lazy] != false
+            @has_unmaterialized_transactions = true
+          else
+            transaction.materialize!
+          end
           @stack.push(transaction)
-          @has_unmaterialized_transactions = true if @connection.supports_lazy_transactions?
           transaction
         end
       end

activerecord/lib/active_record/test_fixtures.rb

@@ -122,7 +122,7 @@ module ActiveRecord
         # Begin transactions for connections already established
         @fixture_connections = enlist_fixture_connections
         @fixture_connections.each do |connection|
-          connection.begin_transaction joinable: false
+          connection.begin_transaction joinable: false, _lazy: false
           connection.pool.lock_thread = true if lock_threads
         end
 
@@ -138,7 +138,7 @@ module ActiveRecord
             end
 
             if connection && !@fixture_connections.include?(connection)
-              connection.begin_transaction joinable: false
+              connection.begin_transaction joinable: false, _lazy: false
               connection.pool.lock_thread = true if lock_threads
               @fixture_connections << connection
             end

activerecord/test/cases/fixtures_test.rb

@@ -924,7 +924,7 @@ class TransactionalFixturesOnConnectionNotification < ActiveRecord::TestCase
       def lock_thread=(lock_thread); end
     end.new
 
-    assert_called_with(connection, :begin_transaction, [joinable: false]) do
+    assert_called_with(connection, :begin_transaction, [joinable: false, _lazy: false]) do
       fire_connection_notification(connection)
     end
   end