Add test for stale seq in merge delta combining.

merge_read_item() fails to update found->seq when combining delta items
from multiple finalized log trees. Add a test case to replicate the
conditions of this issue.

Each of 5 mounts sets totl value 1 on 2500 shared keys, giving an
expected total of 5 per key.  Any total > 5 proves double-counting
from a stale seq.

The log_merge_force_partial trigger forces many partial merges per
cycle, creating the conditions where stale-seq items get spliced into
fs_root while finalized logs still exist.  Parallel readers on all
mounts race against this window to detect double-counted values.

Signed-off-by: Auke Kok <auke.kok@versity.com>

tests/sequence

@@ -27,6 +27,7 @@ simple-xattr-unit.sh
 retention-basic.sh
 totl-xattr-tag.sh
 quota.sh
+totl-merge-read.sh
 lock-refleak.sh
 lock-shrink-consistency.sh
 lock-shrink-read-race.sh