variablesizecommunicatortest does not check the communicated data

It currently sends some data around, and it prints the communicated data. However, nobody looks at the output of test programs anymore, in particular since ctest hides that output as long as the test returns success.

variablesizecommunicatortest should:

• check the data values on the receiving side (e.g. in scatter())
• check that all necessary data is collected in gather()
• check that all sent data is received in scatter()