Extend tests triggering case ad-hoc tasks

Right now, there are multiple tests covering ad-hoc task triggering in the following test classes:

• CaseRuntimeDataServiceImplTest
• CaseServiceImplTest
• CarInsuranceClaimCaseTest

However, some test scenarios are missing and should be implemented, namely:

• triggering ad-hoc tasks from not active stage (should not be allowed)
• triggering the same ad-hoc task multiple times