Use valid data in test fixtures

[nicoweidner]

Currently, test data used in fixtures does not pass validation. While not really necessary, it would be nice to use test data that passes our own validation.
See https://github.com/spdx/tools-python/pull/379/files#diff-5dbc88d6e5c3615caf10e32a9d6fc6ff683f5b5814948928cb84c3ab91c038b6

While at it, those fixtures should be unified with https://github.com/spdx/tools-python/blob/refactor-python-tools/tests/valid_defaults.py

[meretp]

fixed with #409

[meretp]

reopen this as this TODO can't be fixed yet, turning the validation leads to a failing test.

[meretp]

fixed with #417