Improve test coverage

[crandmck]

Perhaps add some performance or load testing as well.

[tmathern]

This has be done/has happened since the issue was opened:

• increased test coverage: see https://github.com/contentauth/c2pa-python/tree/main/tests (and the tests run on each PR too);
• added benchmark (for load/performance): see https://github.com/contentauth/c2pa-python/blob/main/tests/benchmark.py.

[crandmck]

As noted, tests has been added.