Add integration tests

[luhring]

This extension would benefit greatly from adding integration tests. Especially since such a large majority of functionality depends on crossing process boundaries (disk I/O, network I/O, the grype child process), integration tests would add a lot of confidence that the extension is delivering the functionality we expected.

VS Code has created a method of running integration tests on extensions: https://code.visualstudio.com/api/working-with-extensions/testing-extension

We should develop tests that deliver a high amount of value, and we should ensure these tests are running in at least one of our automated pipelines.