refactor: use sync.WaitGroup.Go in tests (#26671)

Migrate `wg.Add(1); go func() { defer wg.Done(); ... }()` to
`wg.Go(func() { ... })` in tests.

Where the prior pattern passed the loop variable explicitly via a
closure parameter (`go func(id int) { ... }(i)`), drop the parameter and
reference the loop variable directly. Per-iteration loop variables since
Go 1.22 make this safe.

coderd/workspacestats/tracker_test.go

@@ -87,11 +87,9 @@ func TestTracker(t *testing.T) {
 	var wg sync.WaitGroup
 	count = 0
 	for i := 0; i < len(ids); i++ {
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
+		wg.Go(func() {
 			tickCh <- now
-		}()
+		})
 		wut.Add(ids[i])
 	}
 
@@ -173,18 +171,14 @@ func TestTracker_MultipleInstances(t *testing.T) {
 	nowB := now.Add(2 * time.Minute)
 	var wg sync.WaitGroup
 	var flushedA, flushedB int
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
+	wg.Go(func() {
 		wuTickA <- nowA
 		flushedA = <-wuFlushA
-	}()
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
+	})
+	wg.Go(func() {
 		wuTickB <- nowB
 		flushedB = <-wuFlushB
-	}()
+	})
 	wg.Wait()
 
 	// We expect 5 flushed IDs each