Soften the comparison of relationships in generation testcase7

[meretp]

Working on #36 I encountered the following problem:
Testcase 7 fails with the file generated by the tools-python because the relationship SPDXRef-fileB DESCRIBED_BY SPDXRef-DOCUMENT is missing. The tools-python don't write this relationship to the generated file as this information is already expressed by the documentDescribes tag (at least for json, yaml and xml). As this is in principle valid, the testbed shouldn't explicitly check if the relationships are equal, it should rather check if the information (SPDXRef-fileB is described by the doucment) is present, either through the relationship or the documentDescribes`-tag.