uucore: error on negative interval in parse_time

Return an error when a negative interval is provided as the argument
to `uucore::parse_time::from_str()`, since a `Duration` should only be
non-negative.

tests/by-util/test_sleep.rs

@@ -149,3 +149,11 @@ fn test_sum_overflow() {
         .no_stderr()
         .no_stdout();
 }
+
+#[test]
+fn test_negative_interval() {
+    new_ucmd!()
+        .args(&["--", "-1"])
+        .fails()
+        .usage_error("invalid time interval '-1'");
+}