MADRID—Rafael Nadal tested positive for the coronavirus after playing in an exhibition tournament in Abu Dhabi, the Spaniard said Monday. Nadal wrote on Twitter that the positive result came from a PCR test he took after arriving in Spain. Nadal said he is enduring “unpleasant moments” but hopes to improve “little by little.” He is in isolation at home and everyone who has been in contact with him was informed of his positive result. Among those possibly in contact with Nadal was Spain’s former monarch, Juan Carlos I, who has been living in the United Arab Emirates since allegations of financial scandals swirled and embarrassed the Spanish royal household last year. According to Spain’s El Mundo newspaper, Nadal and Juan Carlos, who is 83, shared a meal on Saturday. The paper also published photos of both of them posing together without masks. Nadal said he had been tested every two …