BEIRUT — Syrian government forces and their allies regained control Saturday of a predominantly Christian central town that sleeper cells of the Islamic State captured late last month. The government-run Syrian Central Military Media said “the Syrian army and its allies have restored security and stability to Qaryatayn” after clearing the town of out Islamic State fighters. The capture of Qaryatayn came after nearly three weeks of fighting that saw Islamic State capture areas it had earlier lost in an offensive by Syrian government forces and Iranian-backed militiamen under the cover of Russian air strikes.