This patch creates a set of tests that should be true for all implementations of Chonk and CompactableChonk, which we can share with the SQLite implementation in corp.
It includes all the existing tests, plus a test for LastActiveAncestor which was in corp but not in oss.
Updates https://github.com/tailscale/corp/issues/33465
Signed-off-by: Alex Chan <alexc@tailscale.com>
