[ticket/12657] Add a test file for base case

PHPBB3-12657
[ticket/12657] Fix name temporary directory used by test file

PHPBB3-12657
[ticket/12657] Remove unused global variables

PHPBB3-12657
[ticket/12657] The test does not need to depend on the database

PHPBB3-12657
[ticket/12657] Fix the order of the parameters for the constructor

PHPBB3-12657
[ticket/12657] Fix unit tests

PHPBB3-12657
Merge pull request #2545 from ptitlazy/ticket/12657

[ticket/12657] Add a test file for base case of command cache:purge