test: adjust a test to work with js/wasm's background goroutine

Fixes #34768

Change-Id: Ic73591f620cdee5bc7203483902e6ba98d2c442b
Reviewed-on: https://go-review.googlesource.com/c/go/+/200438Reviewed-by: Bryan C. Mills <bcmills@google.com>

test/goprint.go

@@ -9,13 +9,24 @@
 package main
 
 import (
+	"log"
 	"runtime"
 	"time"
 )
 
 func main() {
+	numg0 := runtime.NumGoroutine()
+	deadline := time.Now().Add(10 * time.Second)
 	go println(42, true, false, true, 1.5, "world", (chan int)(nil), []int(nil), (map[string]int)(nil), (func())(nil), byte(255))
-	for runtime.NumGoroutine() > 1 {
-		time.Sleep(10*time.Millisecond)
+	for {
+		numg := runtime.NumGoroutine()
+		if numg > numg0 {
+			if time.Now().After(deadline) {
+				log.Fatalf("%d goroutines > initial %d after deadline", numg, numg0)
+			}
+			runtime.Gosched()
+			continue
+		}
+		break
 	}
 }