spawn_test: prolong termination time to be more tolerant.

pidfd_open(2) can fail on old kernels due to several reasons (e.g.
syscall not supported, O_NONBLOCK flags not supported), in which
case process:wait() falls back to waitpid(2) with backoff (at least
20ms), which may make it longer than 10ms.

Here we add the minimal backoff to 10ms so that the test can pass
on old kernels.

Signed-off-by: Jianyong Chen <baluschch@gmail.com>

tests/unit/spawn_test.cc

@@ -129,10 +129,14 @@ SEASTAR_TEST_CASE(test_spawn_kill) {
             auto* wait_signaled = std::get_if<experimental::process::wait_signaled>(&wait_status);
             BOOST_REQUIRE(wait_signaled != nullptr);
             BOOST_CHECK_EQUAL(wait_signaled->terminating_signal, SIGTERM);
-            // sleep should be terminated in 10ms
             auto end = std::chrono::high_resolution_clock::now();
             auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
-            BOOST_CHECK_LE(ms, 10);
+            // sleep should be terminated in 10ms.
+            // pidfd_open(2) may fail and thus p.wait() falls back to
+            // waitpid(2) with backoff (at least 20ms).
+            // the minimal backoff is added to 10ms, so the test can pass on 
+            // older kernels as well.
+            BOOST_CHECK_LE(ms, 10 + 20);
         });
     });
 }