Fix race on eventloop start in tests

The eventloop provided by k6 is meant to be run until completion
rather than trying to start multiple event loops. This change will
only allow a single eventloop to start until all callbacks have
completed, and only then allow the next eventloop to start. This
issue is only occurring in tests as far as i can tell since in
normal running it doesn't use Start.

k6ext/k6test/vu.go

@@ -2,6 +2,7 @@ package k6test
 
 import (
 	"context"
+	"sync"
 	"testing"
 
 	"github.com/grafana/xk6-browser/k6ext"
@@ -22,14 +23,19 @@ import (
 // VU is a k6 VU instance.
 type VU struct {
 	*k6modulestest.VU
-	Loop *k6eventloop.EventLoop
+	Loop      *k6eventloop.EventLoop
+	runLoopMu sync.Mutex
 }
 
 // ToGojaValue is a convenience method for converting any value to a goja value.
 func (v *VU) ToGojaValue(i interface{}) goja.Value { return v.Runtime().ToValue(i) }
 
 // RunLoop is a convenience method for running fn in the event loop.
 func (v *VU) RunLoop(fn func() error) error {
+	v.runLoopMu.Lock()
+	defer v.runLoopMu.Unlock()
+
+	v.Loop.WaitOnRegistered()
 	return v.Loop.Start(fn)
 }