Test interoperability with the python implementation via subprocess calls

[shibumi]

Please fill in the fields below to submit an issue or feature request. The
more information that is provided, the better.

Description of issue or feature request:
Right now, we have a static test/data directory with some test data in it. I am not sure, if this test data got created with in-toto-python. I would like to suppose, that we add a few tests calling in-toto-python dynamically. That way, we do not mix up in-toto-python generated test/data and our static test data. Furthermore we have some dynamic tests. Thanks to @trishankatdatadog for pointing this out.

Current behavior:
We just test against a static test directory

Expected behavior:
We should be able to test in-toto-python interoperability via calling in-toto-python. The Go TUF implementation is a nice example for such tests.

[lukpueh]

Also see "Cross-implementation interoperability" section in in-toto org roadmap 2021 (WIP, in-toto/specification#37)

[lukpueh]

cc @adityasaky

[adityasaky]

Adding interoperability tests would definitely be great!

[adityasaky]

in-toto/in-toto#563