Example: https://clusterfuzz-external.appspot.com/v2/testcase-detail/6470551652532224

Error: failed to open /mnt/scratch0/clusterfuzz/slave-bot/inputs/fuzzers/libFuzzer/empty/libjpeg-turbo_libjpeg_turbo_fuzzer/afl-testcases. Please use -r if you want to include samples from directories.

We should make recursive search as default as corpuses can have their own subdirectory structure. Thoughts ?