In case of non live test rely on ERP5TypeTestCase to play with MailHost.

In case of live test do not use _setUpDummyMailHost and _restoreMailHost,
which are class based changes, but directly add and remove the object.

Permissions are not checked as it is part of erp5 bt5.

reference of Software Instance does not impact its role's calculation.

Such assertion is already provided by
slapos_assert_hosting_subscription_predecessor alarm.

The category table does not index any non strict link to the computer. It is
not possible in such case to directly use the uid value.

No need to access the host software instance to correctly filter.