Fix TestCase | only "models" works as app_label

[esrehmki]

- in the current implementation only "models" works as app_label
- there is no way to change that during run time as it's hard coded
- the fix introduces another _APP_LABEL global variable that is set
   in the initializer

Enables testing of apps with other labels than "models" in test classes of tortoise/contrib/test
automagically from the TORTOISE_ORM config dict.

Description

Following the design of the file, a new global variable is introduced that is defined during the
call of the initializer function. Later occurences of the app_label function parameter in _setUpDB
member functions, that don't expose the app_label variable, are provided with _APP_LABEL as
default value.

Motivation and Context

App labels other than 'models' were not correctly propagated to the test functions, as 'models'
was hardcoded as app_label in the _setUpDB member functions.

How Has This Been Tested?

Existing tests of the package pass.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.