test_timeout: Add tests for very short timeouts

Note that unlike GNU coreutils, any value > 0 will not be treated
as 0, even if the exponent is very large.

Author: drinkcat

tests/by-util/test_timeout.rs

@@ -125,6 +125,24 @@ fn test_dont_overflow() {
         .no_output();
 }
 
+#[test]
+fn test_dont_underflow() {
+    new_ucmd!()
+        .args(&[".0000000001", "sleep", "1"])
+        .fails_with_code(124)
+        .no_output();
+    new_ucmd!()
+        .args(&["1e-100", "sleep", "1"])
+        .fails_with_code(124)
+        .no_output();
+    // Unlike GNU coreutils, we underflow to 1ns for very short timeouts.
+    // https://debbugs.gnu.org/cgi/bugreport.cgi?bug=77535
+    new_ucmd!()
+        .args(&["1e-18172487393827593258", "sleep", "1"])
+        .fails_with_code(124)
+        .no_output();
+}
+
 #[test]
 fn test_negative_interval() {
     new_ucmd!()