tests: fix and clean up fuzzer script

[eckter]

fixes #3191

Can be reviewed commit per commit, I thought they were small enough not to warrant several PRs (but I can split the PR if that makes it easier).

1. create scenarios when used as a standalone script
2. properly handle and log timeout errors
3. only ignore stdcm 4xx errors when it's a "no path found"
4. update comments and docstrings
5. add optional parameters for stdcm requests