[FIX] tests: restore openerp.tests.common.DB for backwards compatibility

It was removed at 2df9060d97 and broke
tests in community modules that relied on it.

Tests using it should switch to the new get_db_name() method.

Closes #7054

openerp/tests/common.py

@@ -46,6 +46,10 @@ def get_db_name():
     return db
 
 
+# For backwards-compatibility - get_db_name() should be used instead
+DB = get_db_name()
+
+
 def at_install(flag):
     """ Sets the at-install state of a test, the flag is a boolean specifying
     whether the test should (``True``) or should not (``False``) run during