Make SPDX2 model tests less verbose

[armintaenzertng]

In the spdx3 branch, we have developed a model test structure involving a fixture factory that makes initialization tests much less verbose. We should look into whether we can also adopt this structure in the spdx2 model tests.

[HarshvMahawar]

Is this issue still active? if yes, can you please provide some details @armintaenzertng , thanks :)