tr: fix squeeze and complement with two sets (#2485)

tests/by-util/test_tr.rs

@@ -127,6 +127,15 @@ fn test_squeeze_complement() {
         .stdout_is("aaBcDcc");
 }
 
+#[test]
+fn test_squeeze_complement_two_sets() {
+    new_ucmd!()
+        .args(&["-sc", "a", "_"])
+        .pipe_in("test a aa with 3 ___ spaaaces +++") // spell-checker:disable-line
+        .run()
+        .stdout_is("_a_aa_aaa_");
+}
+
 #[test]
 fn test_translate_and_squeeze() {
     new_ucmd!()