I don't have the first clue how long it takes to test a new transit line, but based on some of the photos in this thread I'm wondering if it might be the construction rather than the test schedule that will be the critical piece that drives the opening date.
If, for the sake of argument, they plan to follow two weeks of overnight testing with two weeks of daytime testing and then they're done, that puts them ready to go May 1st-ish. But Bloor Station, for instance, looks more than six weeks away from completion, at least from my armchair vantage point.
The improved weather should speed the contractors up significantly and if they're willing to open their shiny new bauble with construction fencing and hoarding here and there then I'm sure it's quite doable --- presumably the GO platforms at Weston and Bloor can get punted to the back of the queue so the contractors can focus on the UP stuff (if they haven't been directed to already) and even on the exclusively UP infrastructure there are bits and pieces they could open without having ready on Day 1.
Perhaps the testing will take longer than four weeks and much of the above is moot, though.