Improve phpunit test coverage for all parts of the module

Problem/Motivation

With version 2.3.x-dev we introduced more phpunit tests within the module. Right now we have a lot of unit tests covering several parts of the module. Within this issue, we should add more tests to get a higher code coverage.

Remaining tasks

• add more unit tests for tmgmt_deepl and tmgmt_deepl_glosssary module
• add KernelTests were appropriate
• add FunctionalTests were appropriate
• add FunctionalJavascriptTests were appropriate