Use more seeds in non-OSS-Fuzz benchmarks

Hi,

I see in several benchmarks (e.g., LibPNG, LibJPEG-Turbo), there is only one seed (i.e., sample input) in the initial corpus. It would affect the results of several fuzzers, like my AFLSmart structure-aware fuzzer (https://github.com/aflsmart/aflsmart). As we know, the seed corpus quality is very important for (mutational-based) fuzzing. I think it would be good if all benchmarks come with a decent seed corpus.

Best regards,

Thuan