Fuzzing: Initial commit

[AdamKorcz]

Adds initial setup for fuzzing of the image-automation-controller.

To run these fuzzers:

git clone --branch fuzz1 https://github.com/AdamKorcz/image-automation-controller
cd image-automation-controller/fuzz
sudo docker build . --tag flux-fuzzing-image

Next, uncomment one of the fuzzers in the bottom of the Dockerfile and build the image again. This will run the uncommented fuzzer.

Some test helpers are moved to a new file called test_utils.go to allow the fuzzer to use them too.

[pjbgf]

This was superseded by #314. @dholbach do you mind closing this please?

[dholbach]

@pjbgf I moved it forward on the project board - unfortunately I can't close this PR. Thanks for your work on this!