Port existing condition tests

• Ported TimeHeadwayConditionTest from existing_open_scenario_engine/test/TimeHeadwayConditionTest.cpp to the generated engine
• Ported RelativeDistanceConditionTest from existing_open_scenario_engine/test/RelativeDistanceConditionTest.cpp to the generated engine
• Ported UserDefinedValueConditionTest from existing_open_scenario_engine/test/UserDefinedValueConditionTest.cpp to the generated engine
• Some tests had to be adapted for the time being, since the generated engine doesn't implement the checking of preconditions
  • Instead the generated engine either does nothing specific or throws an exception in case some preconditions are not met
• Adapted TimeHeadwayCondition to include pre-condition checks and return false when those conditions are not met