Data flow tests

[fabsx00]

I think we are currently missing initial data flow tests for this frontend similar to:

https://github.com/ShiftLeftSecurity/codepropertygraph/blob/master/c2cpg-tests/src/test/scala/io/shiftleft/c2cpg/querying/C2CpgDataFlowTests.scala

Since the integration of a new language with the data flow engine can sometimes lead to the discovery of problems both in the frontend and the data flow engine, this is an important next step.