expr: Test $ character followed by escaped non-special character

Author: frendsick

tests/by-util/test_expr.rs

@@ -314,6 +314,10 @@ fn test_regex() {
         .args(&["b^$ic", ":", "b^\\$ic"])
         .succeeds()
         .stdout_only("5\n");
+    new_ucmd!()
+        .args(&["a$c", ":", "a$\\c"])
+        .succeeds()
+        .stdout_only("3\n");
     new_ucmd!()
         .args(&["^^^^^^^^^", ":", "^^^"])
         .succeeds()