Because an error prevent creation of res.partner records in the tests due the
'mail' addon adding a required 'notification_email_send' field. This addon does
not depend on 'mail' so the test violates the not-null constraint.
The tests are passing in the normal builds anyway.
Follow OCA/maintainer-quality-tools#187 and use addons/apt/package directive to
install our build dependencies. This allows using container based Travis builds
(which should be faster).