[EPIC] Basic test suite for Engine

Parent: [EPIC] Basic test coverage for all services (aura#175)

Test suite to cover the most important cases.

Please note: The clock.py test cases can be skipped for now, since we will remove this code altogether.

Definition of done

@chrizz and me agreed, that we want to reach a test coverage >= 80%.

Sub Tasks

• Improve test coverage of `aura_engine/base/lang... (#144 - closed)
• Improve test coverage of `aura_engine/base/conf... (#143 - closed)
• Improve test coverage of `aura_engine/plugins/m... (#142 - closed)
• Test cases for "src/aura_engine/core" (#137 - closed)
• Other test cases which look meaningful and improve the overall test coverage