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test: fix flaky gh-5998-one-tx-for-ddl.test.lua

The test expects that any DDL operation aborts **all** concurrent
transactions, but since commit f5f061d051dc ("vinyl: do not abort
unrelated transactions on DDL") this isn't exactly true: transactions
that haven't read/written anything aren't aborted. In the test we expect
a transaction that haven't done anything to be aborted by DDL and it
**is** aborted most of them time but for a different reason: it reads
data that are later modified for `box.schema.user.create()` reads
`box.space._user:max()` to generate an id for the new user first. Since
it reads before writing anything, it has the "read-confirmed" isolation
level hence it's aborted by the transaction creating another user
because the latter updates `box.space._user:max()`. However, sometimes
both users are created and the test fails. This happens if the first
transaction manages to commit before the second one reads the `_user`
system space.

To fix the test and make the transaction creating the second user fail
due to DDL, let's add a read of the `_user` system space before putting
it to sleep. Actually, this even makes the test closer to the "original
test from #5998".

Closes #10444

NO_DOC=test fix
NO_CHANGELOG=test fix

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