The test results had nothing to do with the decision. like Robanker said the real reason is write offs.
it's Purchase Accounting. WB can write off their costs on something like Batgirl as a loss.
New mergers have certain loopholes they can exploit within a window of time to write off certain deals/projects started just before the merger.
Even if it was the reviews, the situation WB was in back when batman and robin came out is very very different from their current situation.
Back then, Batman [a single Intellectual property] wasn't bleeding cash or desperately in debt.
Today WB and it's parent company are heavily in debt and the value of their entire catalogue decreases every time a bad movie is released.
Not remotely comparable situations. They are desperate and have little wriggle room currently.
Cancelling the movie isn't going to lose that much customer trust. They lost that years ago but it will create problems with creatives going forwards most likely.