[FIX] make AccountingTestCase extend HttpCase

Fixes/hides a deadlock when running test_sale_order with either a single
database or a db_filter making a single database visible: the test
causes a report to be printed, which invokes wkhtmltopdf, which tries to
fetch assets using the registry which is being loaded. The asset request
thus waits on a lock which is held by the test which waits on the asset,
deadlocking the whole thing.

Using HttpCase means we're using the test_cr or something and bypassing
the deadlock, instead the asset requests just 404 as they do when using
the nodb routing.

addons/account/tests/account_test_classes.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
-from openerp.tests.common import TransactionCase
+from openerp.tests.common import HttpCase
 
-class AccountingTestCase(TransactionCase):
+class AccountingTestCase(HttpCase):
     """ This class extends the base TransactionCase, in order to test the
     accounting with localization setups. It is configured to run the tests after
     the installation of all modules, and will SKIP TESTS ifit  cannot find an already