Pass an explicit context in the dashboard test

staticcheck SA1012: collector.New tolerates a nil context, but passing one
is still wrong. Caught by make lint after the dashboard test was added.

Author: Paolo Velati

dashboards_test.go

@@ -35,7 +35,7 @@ var exporterMetric = regexp.MustCompile(`\b(?:port|poe|exporter)_[a-z0-9_]+\b`)
 func exposedMetrics(t *testing.T) map[string]bool {
 	t.Helper()
 
-	c := collector.New(nil, collector.Options{
+	c := collector.New(context.Background(), collector.Options{
 		Client: staticClient{},
 		PoE:    true,
 	})