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/notifications/dispatch/smtp_test.go

@@ -445,11 +445,9 @@ func TestSMTP(t *testing.T) {
 
 			// Start mock SMTP server in the background.
 			var wg sync.WaitGroup
-			wg.Add(1)
-			go func() {
-				defer wg.Done()
+			wg.Go(func() {
 				assert.NoError(t, srv.Serve(listen))
-			}()
+			})
 
 			// Wait for the server to become pingable.
 			require.Eventually(t, func() bool {
@@ -590,11 +588,9 @@ func TestSMTPEnvelopeAndHeaders(t *testing.T) {
 			handler := dispatch.NewSMTPHandler(cfg, logger.Named("smtp"))
 
 			var wg sync.WaitGroup
-			wg.Add(1)
-			go func() {
-				defer wg.Done()
+			wg.Go(func() {
 				assert.NoError(t, srv.Serve(listen))
-			}()
+			})
 
 			require.Eventually(t, func() bool {
 				cl, err := smtptest.PingClient(listen, false, false)