[FIX] tests: clean up environments in SavepointCase

The following issue occurs nondeterministically: a test fails when
trying to flush pending updates with a user id that does not exist.
This only happens with `SavepointCase` tests.

Here is the explanation: the method `cr.savepoint()` retrieves an
environment to flush pending updates, and the chosen environment has a
user that was created in another test, and thus no longer exists.  The
problem is that `Environments.envs` contains environments that are no
longer supposed to be used.

The solution consists in, after the test, discarding the environments
that were not present before the test.

closes odoo/odoo#37341

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>

odoo/tests/common.py

@@ -493,6 +493,11 @@ class SavepointCase(SingleTransactionCase):
         super(SavepointCase, self).setUp()
         self._savepoint_id = next(savepoint_seq)
         self.cr.execute('SAVEPOINT test_%d' % self._savepoint_id)
+        # restore environments after the test to avoid invoking flush() with an
+        # invalid environment (inexistent user id) from another test
+        envs = self.env.all.envs
+        self.addCleanup(envs.update, list(envs))
+        self.addCleanup(envs.clear)
 
     def tearDown(self):
         self.cr.execute('ROLLBACK TO SAVEPOINT test_%d' % self._savepoint_id)