[ticket/11600] Increase code test coverage of avatar manager

Currently, not all methods and their possible behaviors are tested with unit tests.
Add them for better code coverage.

PHPBB3-11600
[ŧicket/11600] Split get driver tests into tests for all and only enabled ones

PHPBB3-11600
[ticket/11600] Use local variable for $user in test_localize_errors

PHPBB3-11600
[ticket/11600] Use lowercase null and remove duplicate test cases

PHPBB3-11600
Merge branch 'develop' of https://github.com/phpbb/phpbb3 into ticket/11600
[ticket/11600] Remove duplicate test case

PHPBB3-11600
Merge pull request #1473 from marc1706/ticket/11600

[ticket/11600] Increase code test coverage of avatar manager