fuzz: Add fuzzing harness for LoadMempool(...) and DumpMempool(...)

Summary:

tests: Set errno in FuzzedFileProvider. Implement seek(..., ..., SEEK_END).

validation: Make DumpMempool(...) and LoadMempool(...) easier to test/fuzz/mock

tests: Add fuzzing harness for LoadMempool(...) and DumpMempool(...)

This is a backport of core#19259

With minor changes from other PRs backported out of sequence or not yet backported:

• core#21866: ::Chainstateactive() no longer exists
• core#25290: mempool constructor now takes a MemPoolOptions parameter
• core#19065: ConsumeTime helper function

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15699