Will Goose & MSH's annual COVID-19 insurance pay for the mandatory testing in place for all travellers returning to Canada?

No, our COVID-19 coverage will not pay for such COVID-19 PCR testing. The coverage is designed for emergency medical expenses arising after a positive diagnosis of COVID-19, not for testing purposes prior to being diagnosed. 

More information on what it does cover can be found here:

Goose & MSH's Covid-19 Annual Pandemic plan for Canadian residents