sql/schemachanger: improve performance of TestBackupSuccess

Previously, the successful backup tests would for each stage of a schema
change would run the full schema change, taking a backup at the target
stage. This patch modifies the TestBackupSuccess variants of the schema
changer test to start the server once, take all the backups and then
restore the stages we should be testing.

Testing show around a ~30% improvement with this change in execution
time.

Fixes: #152076

Release note: None

Author: fqazi