tests: execute `addCleanup` cleanups with ZODB connection
Jérome Perrin authored
unittest executes the cleanups after `tearDown`, after the ZODB
connection is closed, so accessing database objects cause errors.

According to python unittest documentation, it is safe to call
`doCleanups` ourselves when we need the cleanup to be executed earlier,
this is a typical case where we want the cleanup to be called before
closing the database connections.
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