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.

1 files changed, 5 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb b/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb
index 112f376d0a..c9e84e48cc 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb
+++ b/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