Catherine Hickland, who plays Tess on ABC's Loving, used to confine her singing to the shower. But after seeing Les Miserables 17 times, Hickland, 38, who hadn't so much as sung karaoke, decided she wanted to become a songbird and on Broadway, no less. Even her husband, All My Children's Michael Knight (Tad), was skeptical. ''He said, 'You want to sing on Broadway? That's nice, honey,' '' Hickland recalls. But last week, after months of lessons, she began a three-month stint as Fantine in Les Miz. ''This is without a doubt the most fun I've ever had,'' she says, ''and the hardest thing I've ever done.'' How will the singing soap star measure her success on the Great White Way? ''As long as I don't fall into the orchestra pit, I'll be happy.''