Yeah I am a PR so there is no need for travel insurance when I come into Canada. Not sure if you were talking about that w.r.t. the waiting period.No one knows when and where you would get infected hence there is no safe travel either to Canada or to US.
ON has no waiting period if you are moving. If you are visiting then anyway you need to have travel insurance which would cover for it - whether its a pandemic or not.
Why would you get stuck in US since you are going to get negative report from India before boarding hence you can travel within 72 hours and cross the border as well.
If you are worried about the travel itself then you need to test the option of requesting cic to allow you to attend the exam from abroad. Decide soon and request since cic will take 2 weeks to respond.
And as @Das67 said, I would need to get an RT-PCR from the US before entering Canada as per the new requirements to close the loopholes.
Thanks for the suggestion about requesting CIC though, I will send them an email right away.