[MERGE] mail: clean and move tests

This commit cleans mail tests in order to have less low-level tests and
to improve code coverage. It improves the tests of features and reduces
field-level checks. Some tests depending on implementation details are
wrapped inside methods. This way if implementation changes most tests
are still valid once assert methods are updated.

Common classes, tools and models are improved. More test models exist
in order to be able to test several features and prepare land to add
performance test. Each set of tests is updated in its own commit.

Finally tests are moved to test_mail. This way tables linked to test
models are not present in production database. Existing test_performance
module is split so that there is tests for base and tests for mail.

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