main: add coroutine/sleep tests

testdata/coroutines.go

@@ -0,0 +1,18 @@
+package main
+
+import "time"
+
+func main() {
+	println("main 1")
+	go sub()
+	time.Sleep(1 * time.Millisecond)
+	println("main 2")
+	time.Sleep(2 * time.Millisecond)
+	println("main 3")
+}
+
+func sub() {
+	println("sub 1")
+	time.Sleep(2 * time.Millisecond)
+	println("sub 2")
+}

testdata/coroutines.txt

@@ -0,0 +1,5 @@
+main 1
+sub 1
+main 2
+sub 2
+main 3