test: add test for #7394 (`space:count` bug in memtx MVCC)
The memtx transaction manager MVCC invariant violation described in
c8eccfbb could actually lead to a bug described in #7394, since
`space:count` inherently relies on this invariant: add a test for this
case.
Closes #7394
NO_CHANGELOG=<bug was fixed in c8eccfbb>
NO_DOC=bugfix