Each database stored on Altas shared tier is assigned a database name of the form [tenantId]_[originalDatabaseName]. To make sure that the database prefix for a tenant migration always corresponds to a tenant id, we should include "_" in this check in TenantMigrationAccessBlockerByPrefix::getTenantMigrationAccessBlocker (i.e.  change it to dbName.startsWith(dbPrefix + "_")). 

Each existing jstest for tenant migrations that runs donorStartMigration with 'databasePrefix' equals to the name of the database that it intends to migrate will need to be updated.