OnDiskCorpus actually only evicts the input from memory, not the entire Testcase

From the docs of `OnDiskCorpus`:
https://github.com/AFLplusplus/LibAFL/blob/2842f95e2aa4e0c665df3c99f1e46a5f4db52fbd/libafl/src/corpus/ondisk.rs#L1-L6

`OnDiskCorpus` uses `CachedOnDiskCorpus` under the hood:
https://github.com/AFLplusplus/LibAFL/blob/2842f95e2aa4e0c665df3c99f1e46a5f4db52fbd/libafl/src/corpus/cached.rs#L1

However, both these docs are wrong: `CachedOnDiskCorpus` only ever evicts the actual input from memory, the rest of the Testcase is kept in memory, including all metadata.

We should either implement what the docs say (not sure about the performance implications though), or at least fix the docs.

(Disclaimer: I'm only about 95% sure.)