tenant_migration_timeseries_retryable_write_oplog_cloning.js should get donorDoc before being garbage collected

There is a race condition in tenant_migration_timeseries_retryable_write_oplog_cloning.js. The test calls tenantMigrationTest.runMigraition() which automatically forgets migration and so the donorDoc could already be garbage collected before the test tries to lookup for the donorDoc and thus returns null.