Improve OS independence re unit tests #2633

[phillxnet]

• Add missing mock in test_commands.py re enable_quota() & get_unlocked_luks_containers_uuids().
• add missing mock in test_email_client.py re update_master().
• add missing mock in test_samba.py re refresh_smb_discovery().
• add missing mocks in test_snmp.py re systemd interaction.
• add missing mock in test_network.py re nmcli call.
• add missing mock in test_snapshot.py re refresh_nfs_exports.
• add missing mock in test_config_backup.py re file expectation.
• add missing mock in test_disks.py re dmsetup call.
• revise & update test_user.py re fixtures use. Re-enables a number of remarked-out user tests awaiting this revision. Improves readability, scope, and specificity re test names.

Includes improved instructions on running each test set for all test files modified.

Fixes #2633

[phillxnet]

The testing for this pull request is to be done against the rockstor-rpmbuid work done against issue:

"Improve BuildRequires use": rockstor/rockstor-rpmbuild#28

As this pr is a spin-off from work in-progress done against the above issue which aims to reduce our BuildRequires: ergo the test improvements here that otherwise created false rpmbuild dependencies due to inappropriate OS sensitivity during test rurns.

[phillxnet]

See the following rockstor-rpmbuild pull request comment re test builds including this patch to rockstor-core:
rockstor/rockstor-rpmbuild#39 (comment)

We now expect the following output from a %check rpmbuild scriptlet:

+ poetry run django-admin test
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Ran 262 tests in 49.090s