The function `vy_space_build_index`, which builds a new index on DDL,
calls `vy_scheduler_dump` on completion. If there's a checkpoint in
progress, the latter will wait on `vy_scheduler::dump_cond` until
`vy_scheduler::checkpoint_in_progress` is cleared. The problem is
`vy_scheduler_end_checkpoint` doesn't broadcast `dump_cond` when it
clears the flag. Usually, everything works fine because the condition
variable is broadcast on any dump completion, and vinyl checkpoint
implies a dump, but under certain conditions this may lead to a fiber
hang. Let's broadcast `dump_cond` in `vy_scheduler_end_checkpoint`
to be on the safe side.

While we are at it, let's also inject a dump delay to the original
test to make it more robust.

Closes #10267
Follow-up #10234

NO_DOC=bug fix