The test fails during the final `z_sendmany`, because it is selecting a
note that was detected before restarting the node. Because we force the
wallet to call `SetBestChain` on every block, the wallet doesn't need to
rescan on restart, and thus doesn't repopulate the `position` field of
the in-memory note.
This issue went unnoticed in existing tests that exercise node restarts,
because the RPC tests are fast enough that they never pass the 10-minute
timeout for writing the wallet state. This commit adds a regtest-only
config option that disables the lazy writing.
Kris Nuttycombe
str4d
Jack Grigg