Do not use a 0:0 absolute timeout in timer test.

Document that an absolute timeout of 0:0 is not supported.

doc/html/timer.html

@@ -82,7 +82,10 @@ <h2>Details and Usage</h2>
 is supported, be aware that it rarely makes sense to do so; the timer is not guaranteed to expire after
 the specified interval (if the system clock is adjusted, the adjustment will also be applied to the
 timeout). Use a timer against the <i class="code-name">MONOTONIC</i> clock if you need a specific interval
-of time.</p> 
+of time.</p>
+
+<p>An absolute timeout of 0:0 is not supported (some timer backends use it internally to disable the
+associated system timer).</p>
 
 <h3>Subclassing timer</h3>
 

tests/dasynq-tests.cc

@@ -1057,7 +1057,7 @@ void ftest_timers()
     };
 
     my_timer timer_1;
-    struct timespec timeout_1 = { .tv_sec = 0, .tv_nsec = 0 };
+    struct timespec timeout_1 = { .tv_sec = 0, .tv_nsec = 1 };
     timer_1.add_timer(my_loop, clock_type::MONOTONIC);
     timer_1.arm_timer(my_loop, timeout_1);