sql/tests: avoid contention in RSG test

TestRandomSyntaxSchemaChangeColumn would always operate on the same
table, which would cause many schema changes to get queued up, since
everything happens concurrently. This isn't even that realistic, so now
it does the schema change on one of 50 tables randomly.

Release note: None

pkg/sql/tests/rsg_test.go

@@ -491,6 +491,7 @@ func TestRandomSyntaxSchemaChangeColumn(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)
 
+	numTables := *flagRSGGoRoutines
 	roots := []string{
 		"alter_table_cmd",
 	}
@@ -499,13 +500,18 @@ func TestRandomSyntaxSchemaChangeColumn(t *testing.T) {
 		if err := db.exec(t, ctx, "SET CLUSTER SETTING sql.catalog.descriptor_lease_duration = '30s'"); err != nil {
 			return err
 		}
-		return db.exec(t, ctx, `
-			CREATE DATABASE ident;
-			CREATE TABLE ident.ident (ident decimal);
-		`)
+		if err := db.exec(t, ctx, `CREATE DATABASE ident;`); err != nil {
+			return err
+		}
+		for i := 0; i < numTables; i++ {
+			if err := db.exec(t, ctx, fmt.Sprintf(`CREATE TABLE ident.ident%d (ident decimal);`, i)); err != nil {
+				return err
+			}
+		}
+		return nil
 	}, func(ctx context.Context, db *verifyFormatDB, r *rsg.RSG) error {
 		n := r.Intn(len(roots))
-		s := fmt.Sprintf("ALTER TABLE ident.ident %s", r.Generate(roots[n], 500))
+		s := fmt.Sprintf("ALTER TABLE ident.ident%d %s", r.Intn(numTables), r.Generate(roots[n], 500))
 		// Execute with a resettable timeout, where we allow up to N go-routines worth
 		// of resets. This should be the maximum theoretical time we can get
 		// stuck behind other work.