testdata: fix flaky goroutines test

The goroutines test had two races under thread-based scheduling:

1. "main 2" vs "sub 2": both printed around 100-200ms from start,
   racing on loaded systems. Fixed by using channel synchronization
   between main and sub() instead of relying on sleep timing.

2. "slept inside func pointer" vs "slept inside closure": both
   goroutines launched by sleepFuncValue slept 1ms then printed,
   with only a 1ms gap between launches. Fixed by waiting for each
   goroutine to complete via a done channel before starting the next.

The test still exercises goroutine creation, function value calls,
closure captures, and channel communication.

Addresses tinygo-org/tinygo issue 4965.

Author: jakebailey

testdata/goroutines.go

@@ -11,12 +11,15 @@ func init() {
 	time.Sleep(1 * time.Millisecond)
 }
 
+var subCh = make(chan struct{})
+var sleepDone = make(chan bool)
+
 func main() {
 	println("main 1")
 	go sub()
-	time.Sleep(100 * time.Millisecond)
+	<-subCh // wait for sub to print "sub 1"
 	println("main 2")
-	time.Sleep(200 * time.Millisecond)
+	<-subCh // wait for sub to print "sub 2"
 	println("main 3")
 
 	// Await a blocking call.
@@ -41,15 +44,16 @@ func main() {
 	sleepFuncValue(func(x int) {
 		time.Sleep(1 * time.Millisecond)
 		println("slept inside func pointer", x)
+		sleepDone <- true
 	})
-	time.Sleep(1 * time.Millisecond)
+	<-sleepDone
 	n := 20
 	sleepFuncValue(func(x int) {
 		time.Sleep(1 * time.Millisecond)
 		println("slept inside closure, with value:", n, x)
+		sleepDone <- true
 	})
-
-	time.Sleep(2 * time.Millisecond)
+	<-sleepDone
 
 	var x int
 	go func() {
@@ -103,8 +107,10 @@ func acquire(m *sync.Mutex, wg *sync.WaitGroup) {
 
 func sub() {
 	println("sub 1")
-	time.Sleep(200 * time.Millisecond)
+	subCh <- struct{}{}
+	time.Sleep(150 * time.Millisecond)
 	println("sub 2")
+	subCh <- struct{}{}
 }
 
 func wait() {