[FIX] account: fix test on closed period since account_lock module

Module account_lock has been introduced by:
https://github.com/odoo/odoo/commit/2eb344f23b3a9daa8e7c7ddaead145a8b05b39bf

A new constrains appears on the lock dates: their must not be set
after the last day of the previous month.
Then, it breaks the test on closed period that set the lock date 'yesterday'.

addons/account/tests/test_account_move_closed_period.py

 from openerp.addons.account.tests.account_test_classes import AccountingTestCase
 from openerp.osv.orm import except_orm
-from datetime import datetime, timedelta
+from datetime import datetime
+from dateutil.relativedelta import relativedelta
+from calendar import monthrange
 from openerp.tools import DEFAULT_SERVER_DATE_FORMAT
 
 class TestPeriodState(AccountingTestCase):
@@ -10,26 +12,26 @@ class TestPeriodState(AccountingTestCase):
 
     def setUp(self):
         super(TestPeriodState, self).setUp()
-        cr, uid = self.cr, self.uid
         self.user_id = self.env['res.users'].browse(self.uid)
-        self.day_before_yesterday = datetime.now() - timedelta(2)
-        self.yesterday = datetime.now() - timedelta(1)
-        self.yesterday_str = self.yesterday.strftime(DEFAULT_SERVER_DATE_FORMAT)
+
+        last_day_month = datetime.now() - relativedelta(months=1)
+        last_day_month = last_day_month.replace(day=monthrange(last_day_month.year, last_day_month.month)[1])
+        self.last_day_month_str = last_day_month.strftime(DEFAULT_SERVER_DATE_FORMAT)
+
         #make sure there is no unposted entry
-        draft_entries = self.env['account.move'].search([('date', '<=', self.yesterday_str), ('state', '=', 'draft')])
+        draft_entries = self.env['account.move'].search([('date', '<=', self.last_day_month_str), ('state', '=', 'draft')])
         if draft_entries:
             draft_entries.post()
-        self.user_id.company_id.write({'fiscalyear_lock_date': self.yesterday_str})
+        self.user_id.company_id.fiscalyear_lock_date = self.last_day_month_str
         self.sale_journal_id = self.env['account.journal'].search([('type', '=', 'sale')])[0]
         self.account_id = self.env['account.account'].search([('internal_type', '=', 'receivable')])[0]
 
     def test_period_state(self):
-        cr, uid = self.cr, self.uid
         with self.assertRaises(except_orm):
             move = self.env['account.move'].create({
                 'name': '/',
                 'journal_id': self.sale_journal_id.id,
-                'date': self.day_before_yesterday.strftime(DEFAULT_SERVER_DATE_FORMAT),
+                'date': self.last_day_month_str,
                 'line_ids': [(0, 0, {
                         'name': 'foo',
                         'debit': 10,