[C++] convert ImmutableMessageTest class to fixture
[C++] convert ImmutableMessageTest class to fixture

Change-Id: I4eee1a01badccc80327d0675d84c3afd3d6437ce

bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr