If they can counterfeit passports they can counterfeit negative Covid test certificates just as easily.
The BBC just posted this:
“Europol, the EU’s law enforcement agency, has issued a warning that criminals are selling fake negative Covid-19 test certificates. The fear is this could enable people to cheat travel restrictions.
It comes as an investigation by BBC News has found falsified NHS certificates for sale online. There are also ones from clinics in Nigeria sold in forums used by organised crime groups”
See: https://www.bbc.co.uk/news/av/uk-55890168