schemachanger: fix bug in concurrency test framework

The newly-rewritten TestConcurrentDeclarativeSchemaChanges test had
a concurrency bug in it, which caused it to fail in cases where the
two schema changes actually take place serially. It's possible for the
later one to start after the earlier one has already finished.

Fixes: cockroachdb#106732.

Release note: None

pkg/sql/schemachanger/schemachanger_test.go

@@ -140,32 +140,28 @@ func TestConcurrentDeclarativeSchemaChanges(t *testing.T) {
 	}
 
 	backfillNotif, continueNotif := initBackfillNotification()
-	var wg sync.WaitGroup
-	wg.Add(1)
-	go func() {
-		if _, err := sqlDB.Exec(`CREATE INDEX i ON t.test (v)`); err != nil {
-			t.Error(err)
-		}
-		wg.Done()
-	}()
+	g := ctxgroup.WithContext(ctx)
+	g.Go(func() error {
+		_, err := sqlDB.Exec(`CREATE INDEX i ON t.test (v)`)
+		return err
+	})
 
-	// Wait until the create index schema change job has started
-	// before kicking off the alter primary key.
+	// Wait until the CREATE INDEX schema change job has started
+	// before kicking off the ALTER PRIMARY KEY.
 	<-backfillNotif
 	require.Zero(t, alterPrimaryKeyBlockedCounter.Load())
-	wg.Add(1)
-	go func() {
-		if _, err := sqlDB.Exec(`ALTER TABLE t.test ALTER PRIMARY KEY USING COLUMNS (k)`); err != nil {
-			t.Error(err)
-		}
-		wg.Done()
-	}()
+	g.Go(func() error {
+		_, err := sqlDB.Exec(`ALTER TABLE t.test ALTER PRIMARY KEY USING COLUMNS (k)`)
+		return err
+	})
 
-	// Unblock the create index job.
+	// Unblock the CREATE INDEX, and, by extension, ALTER PRIMARY KEY.
 	continueNotif <- struct{}{}
-	wg.Wait()
 
-	// The ALTER PRIMARY KEY schema change must have been blocked.
+	// Wait for both schema changes to complete.
+	require.NoError(t, g.Wait())
+
+	// The ALTER PRIMARY KEY schema change must have blocked.
 	require.NotZero(t, alterPrimaryKeyBlockedCounter.Load())
 
 	// There should be 5 k/v pairs per row: