The best way in every circumstance will boil down to understanding your client's perspective - including, but not limited to, the extent to which they value time as compared to cost and quality.
It's always tempting for the project manager to paint a rosy picture and not inform the sponsor/client of a risk. That is usually the wrong thing to do because it deprives the sponsor/client of their power to make decisions that steer the project in the right direction on the time/cost/quality spectrum.
The right way will vary, but is probably to present the risk alongside your analysis of the options to mitigate that risk. The analysis should represent your understanding of the client's needs.
For example, produce a table with options:
Option 1 - throw money at the problem - maybe you can bring in extra resource/work extra hours. Pros - gets the job done more quickly, cons - more costly, maybe detracts from quality if you're bringing in non-experts or, if they're inclined toward caring about staff well-being, may add to stress levels.
Option 2 - reduce scope and/or test time. Pros - gets job done more quickly, cons - quality may be diminished.
Option 3 - focus on quality outputs & extend timelines. Pros - high quality project Cons - more time and, therefore, may eat into contingency budget.
Option 4 - maybe you can come up with more out-of-the-box options like a phased delivery?
Then produce a thought-out recommendation to the client based on what you think is most important to them. They may disagree with you, but they will appreciate your efforts to a) give them a choice and b) make the problem & decision as simple as possible.
Hope that helps.