For fuzz tests that need it.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/21866 | core#21866]] [6/12]
https://github.com/bitcoin/bitcoin/pull/21866/commits/ee0ab1e959e0e75e04d87fabae8334ad4656f3e5
Note: src/test/fuzz/validation_load_mempool.cpp has not been created yet in Bitcoin ABC. If/When it will eventually be backported, it will be obvious that this change is needed, as `g_chainman` will no longer exist by then.