Add tests for noexit in the py lib's session_setup for API Fuzzing

Problem

A bug was fixed in session_setup for an edge case around noexit, but no unit tests were added. Testing requires mocking up the scan engine rest api, a larger task that should not hold up release of the bugfix.

Proposal

1. Setup a way to mock the scan engine api for testing
2. Add noexit tests for all known cases

Links

• Bug fix MR https://gitlab.com/gitlab-org/security-products/analyzers/api-fuzzing-src/-/merge_requests/428