CANTON, Mass. — Mitt Romney testified under oath in 1991 that the ex-wife of Staples founder Tom Stemberg got a fair deal in the couple’s 1988 divorce, even though the company shares Maureen Sullivan Stemberg received were valued at a tenth of Staples’ stock price on the day of its initial public offering only a year later. At the time the Stembergs split, Romney suggested, there was little indication that Staples’ value would soon skyrocket. Romney’s testimony in a post-divorce lawsuit brought in 1990 by Sullivan Stemberg was unsealed on Thursday in Norfolk Probate and Family Court at the Globe’s...