interp: run goroutine starts and checks at runtime

This change prevents interp from trying to execute goroutine starts or checks.
This fixes a bug where a goroutine started by an init function would run before the init function.

testdata/goroutines.go

@@ -6,6 +6,12 @@ import (
 	"time"
 )
 
+func init() {
+	println("init")
+	go println("goroutine in init")
+	time.Sleep(1 * time.Millisecond)
+}
+
 func main() {
 	println("main 1")
 	go sub()