check keyspace snapshot time for restore

[pbibra]

Description

Snapshot restores are no longer restoring from the backup before or at the snapshot time. Regression introduced here: vitessio#8824

This is breaking our PITR workflow. v14 is no longer supported upstream so adding the fix in our fork. Opened an issue and PR upstream, but in discussion about the eventual deprecation of snapshot keyspaces in favor of incremental backups/restores.

Testing

Before, PITR tablets always restoring to latest backup

After:

pbibra@tablet-iad-dev-pitrincd1234loadtest-80-00-fsmq:~$ cat /mnt/vitess/vttablet/log/vttablet.log | grep found
I0718 12:08:11.547793   30934 backup.go:256] Restore: No restore_in_progress file found, checking no existing data is present
I0718 12:08:11.913137   30934 backupengine.go:223] Restore: found backup loadtest/80- 2023-07-15.110009.us_east_1c-0173597869 to restore using the specified timestamp of '2023-07-15.150000'

Also modified the existing TestRestoreImpl to properly check for custom backup times.

Before:

After: CI passes

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