Workaround: omit github.com/mattermost/mattermost-server/v5/app module from TE packages-under-test

For some reason, the 'app' module tests appear to attempt to access a SQLStore on 127.0.0.1 (localhost) despite the presence of TEST_DATABASE-prefixed environment variables during CI runs with .gitlab-ci.collabora.yml.

Until we trace the cause of this, temporarily disable this module during testing.
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