Refactor fuzzing implementations to support go native feature

[pjbgf]

Flux has recently implemented an initial coverage of fuzzing across its repositories (xref https://github.com/fluxcd/flux2/issues/2002). At the time of implementation golang did not offer native fuzz testing support, which is yet to be released in golang 1.18.

The proposed refactoring would:

• Build fuzzers using compile_native_go_fuzzer instead of compile_go_fuzzer.
• Review need to depend on github.com/AdaLogics/go-fuzz-headers.
• Better integrate with existing tests and reuse private funcs used within controller testing.
• Reuse testenv instances across multiple executions to decrease test setup time.

Repositories to be refactored:

• [x] source-controller
• [x] kustomize-controller
• [x] helm-controller
• [x] notification-controller
• [x] pkg
• [x] image-reflector-controller
• [ ] image-automation-controller

Dependencies for this work:

• [x] oss-fuzz to support golang native fuzzing
• [x] golang 1.18 to be officially released
• [x] Target repository to be upgraded to golang 1.18

[ilanpillemer]

@pjbgf Go 1.18 has officially landed.

[pjbgf]

@ilanpillemer thanks for the nudge, issue updated accordingly.

We probably will need some upstream dependencies to move on to Go 1.18 before we can do the same - last time around the key dependency was Kubernetes-related if my memory serves me well.

[pjbgf]

Source controller is the only outstanding controller, which will be dealt with after https://github.com/fluxcd/source-controller/pull/879 is merged.

[pjbgf]

Now that https://github.com/fluxcd/source-controller/pull/944 has been merged, this is no longer blocked and the SC work is now set to "up-next".