feat: add test 6.1.31

[peinjoh]

resolves #114

[peinjoh]

@tschmidtb51 One remark on this: Test case 6-1-31-12 contains a branch with name after-eight, but is a valid case. This was also highlighted by @oxisto here: oasis-tcs/csaf#825

This was remediated in version 2.1 of the standard, which introduces the need for tokenization before checking for the "forbidden" words (i.e. before, after, ...) based on the suggestion in the issue above.

Still, this test case being valid contradicts the standard, as after-eight contains after. I implemented the test by using the tokenization for both 2.0 and 2.1 documents, is this the intended / correct implementation specifically for CSAF 2.0 documents?