Ensure that test target uses all test data files

[senier]

Currently if we add test data under tests/data, this files may just not be used (e.g. tests/data/ipv6 is not being used!).

Add a check to the test case that every regular file except README files are being used.

[jklmnn]

In Componolit/RecordFlux-specifications#80 it might be necessary to restrict the allowed file extensions for test data (e.g. only allow .raw). This is required for the *.yaml files that are required to provide message parameters. Additionally any other file should create a warning about an unused/unsupported file.