Count each epoll test deadline from its own arm call

The poller test recorded every initial deadline as the same start time
plus the delay, while the poller reads the clock at each arm call; when
arming the 2,000 timers took over 1 ms, the recorded order could differ
from the delivered one by more than the assertion's slack. Each deadline
now counts from the time read just before its scrp_timer_arm call, as
the re-arm loop's do, and a 3 ms pause halfway through arming keeps the
test exercising that case.

packages/runtime/src/scr_loop_epoll.test.c

@@ -38,10 +38,17 @@ int main(void) {
   assert(p != NULL);
   srand(7);
   double start = now_ms();
+  /* Each deadline counts from its own arm call: arming takes time, here
+   * stretched by a pause halfway, and the poller reads the clock at each
+   * call. */
   for (int i = 0; i < KEYS; i++) {
+    if (i == KEYS / 2)
+      for (double until = now_ms() + 3; now_ms() < until;) {
+      }
     double ms = 5 + rand() % 120;
+    double t = now_ms();
     assert(scrp_timer_arm(p, &keys[i], ms, &keys[i]));
-    deadline[i] = start + ms;
+    deadline[i] = t + ms;
     state[i] = 1;
   }
   /* re-arm a third later and earlier, cancel a third (the heap and the key