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/x/chatd/configcache_internal_test.go

@@ -416,12 +416,10 @@ func TestConfigCache_Singleflight(t *testing.T) {
 	var wg sync.WaitGroup
 	start := make(chan struct{})
 	for i := 0; i < callers; i++ {
-		wg.Add(1)
-		go func(i int) {
-			defer wg.Done()
+		wg.Go(func() {
 			<-start
 			results[i], errs[i] = cache.EnabledProviders(ctx)
-		}(i)
+		})
 	}
 
 	close(start)

coderd/x/nats/pubsub_test.go

@@ -139,11 +139,9 @@ func TestPubsub(t *testing.T) {
 
 		var first, second error
 		var wg sync.WaitGroup
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
+		wg.Go(func() {
 			first = ps.Close()
-		}()
+		})
 		wg.Wait()
 		second = ps.Close()
 		assert.NoError(t, first)