Sanity check harnesses in the coverage reports and/or builds

[DavidKorczynski]

In the recent large scale experiment some projects got a wrong benchmark target: #618

We could have discovered this doing some form of sanity check in the source code at some stage.

The sample is e.g. hiredis where the benchmarks has:

...
"target_name": "format_command_fuzzer"
"target_path": "/src/hiredis/format_command_fuzzer.c"
"test_files":
- "test_file_path": "/src/hiredis/examples/example-ssl.c"
...

However, because the harness is moved around in build.sh we ended up flagging two possible harness candidates and overwrote the destination of the copy, meaning our harness would not be in the source code: https://github.com/google/oss-fuzz/blob/bf53d7f396930655fa3db2a9b89b7cab9a543b25/projects/hiredis/build.sh#L19

We can do a couple of things:

• better harness target detection. We can probably add some heuristics here.
• sanity check