Fix the from-disk subtest to use a separate node so it builds on a
clean genesis block, rather than the leftover chain from the
in-memory subtest.
Change from a two-way to a three-way block race. The UB in the old
LoadChainTip (mutating nSequenceId, a sort key, while the block is
in setBlockIndexCandidates) corrupts the internal tree structure,
resulting in a failed erase that leaves stale blocks in the set
alongside the tip. With only two competing blocks, this is caught
by libstdc++ but not by libc++. A three-way split triggers the bug
on both implementations.
To trigger CheckBlockIndex (where the crashing assertion is), replace
the restart loop with sending a new block after a single restart.
Adds tests to make sure we are consistent on activating the same chain over
a node restart if two or more candidates have the same work when the node is shutdown
