[IMP] mail: add two performance tests

This commits adds two tests in test_mail performance tests related to
the write operation. Indeed create a often tested but there are less
performance tests related to the write. Those two tests are

 * first one is a simple write on a chatter-enabled model without any
   tracking or specific feature; nothing should happen when performing
   a write in that kind of model;
 * second one is a write on a complex tracking-enabled model linked to
   templates and auto subscription. It allows to have some figures for
   real world use case.

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