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.
This was used to, in effect, manually emulate --start-group/--end-group.
However, we can just order the libraries correctly and avoid specifying
libraries multiple times on the link line.
Note: lld (not ld.bfd) knows how to resolve out-of-order references and
doesn't seem to need the reodering
