Not only does this increase coverage, it is also more correct in that
when ::LoadMempool is called with a mempool and chainstate, it calls
AcceptToMemoryPool with just the chainstate.
AcceptToMemoryPool will then act on the chainstate's mempool via
CChainState::GetMempool, which may be different from the mempool
originally passed to ::LoadMempool. (In this fuzz test's case, it
definitely is different)
Also, move DummyChainstate to its own file since it's now used by the
validation_load_mempool fuzz test to replace CChainState's m_mempool.
