Refactor fuzzing implementations to support go native feature

[pjbgf]

Flux has recently implemented an initial coverage of fuzzing across its repositories (xref #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:

• source-controller
• kustomize-controller
• helm-controller
• notification-controller
• pkg
• image-reflector-controller
• image-automation-controller

Dependencies for this work:

• oss-fuzz to support golang native fuzzing
• golang 1.18 to be officially released
• 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.