workload/tpcc: fix race condition in scattering tables

When iterating over the names of tables to scatter, we were sharing the
loop variable between the goroutines, which is racy and caused a test
failure due to reading a garbage string value for the table name. In
general, this function is not going to scatter each table exactly once.
This PR fixes it by making a copy of the variable per loop iteration.

Release note (bug fix): Fixed a race condition in the `tpcc` workload
with the `--scatter` flag where tables could be scattered multiple times
or not at all.

pkg/workload/tpcc/ddls.go

@@ -226,6 +226,7 @@ func scatterRanges(db *gosql.DB) error {
 
 	var g errgroup.Group
 	for _, table := range tables {
+		table := table
 		g.Go(func() error {
 			sql := fmt.Sprintf(`ALTER TABLE %s SCATTER`, table)
 			if _, err := db.Exec(sql); err != nil {