Move the vu context creation during iteration

The context in the vu is not thread safe outside of the iteration or
the start or end of the vu. By retrieving the vu's context inside the
iteration we are working with the vu's context in a thread safe way.

browser/registry.go

@@ -230,10 +230,9 @@ func newBrowserRegistry(vu k6modules.VU, remote *remoteRegistry, pids *pidRegist
 
 func (r *browserRegistry) handleIterEvents(eventsCh <-chan *k6event.Event, unsubscribeFn func()) {
 	var (
-		ok    bool
-		data  k6event.IterData
-		ctx   = context.Background()
-		vuCtx = k6ext.WithVU(r.vu.Context(), r.vu)
+		ok   bool
+		data k6event.IterData
+		ctx  = context.Background()
 	)
 
 	for e := range eventsCh {
@@ -251,6 +250,12 @@ func (r *browserRegistry) handleIterEvents(eventsCh <-chan *k6event.Event, unsub
 			return
 		}
 
+		// The context in the VU is not thread safe. It can
+		// be safely accessed during an iteration but not
+		// before one is started. This is why it is being
+		// accessed and used here.
+		vuCtx := k6ext.WithVU(r.vu.Context(), r.vu)
+
 		if data, ok = e.Data.(k6event.IterData); !ok {
 			e.Done()
 			k6ext.Abort(vuCtx, "unexpected iteration event data format: %v", e.Data)