Fix validation against amber provenance schema and cleanup the tests

[rbehjati]

Why does the path need to be restored? Does it mean that it affects the execution of other test cases? That would seem problematic, since the execution of the tests would differ whether they are running concurrently or not. Is it possible to make the entire process not depend on the PWD and instead use absolute paths for everything? It would still be possible here to concat PWD with a relative path to obtain an absolute path and use that, but at least that would not mutate global state.

Originally posted by @tiziano88 in #124 (comment)

[rbehjati]

We are planning to fully switch to the generic SLSA provenance format. Once that is done this problem will probably automatically go away.