handle libfuzzer targets failing without dropping crash files

[bmc-msft]

Summary of the Pull Request

Handle libfuzzer targets exiting with an error without recording crash files

PR Checklist

• Applies to work item: handle libfuzzer targets crashing without dropping an a crashing input #57
• CLA signed. If not, go over here and sign the CLI.
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Info on Pull Request

• Moves src/process.rs from onefuzz-supervisor to onefuzz
• Keeps a copy of stderr in case the fuzzer crashes without dropping crash file
• Adds onefuzz-agent debug libfuzzer_fuzz debug subcommand to test libfuzzer targets locally

Validation Steps Performed

• Deploy a regular libfuzzer task and note it detects crashes as appropriate
• Deploy a target that does raise(SIGKILL); and note that we mark the task is failed