Stop a finished run's clock

Run.snapshot() computed elapsed_s from the current time on every call,
whether the run had ended or not. The page shows the last run until the
next one starts, and it polls, so a finished run's figure went on
counting: the result badge said FAIL beside a number still climbing, and
the run read as still going. On the bench reference a test that ended
after 5082 s was showing 5242 s and rising.

The figure a finished run should carry was already recorded next to it:
finished["duration_s"], fixed when the result was written. snapshot()
now returns that once the run has ended, and the live count only while
it is running.

Host-proven: two unit tests - a finished run's clock reads its duration
and does not move across a poll, a running one's still climbs.

forgetest/forgetest/runner.py

@@ -164,7 +164,12 @@ class Run:
             notice = dict(self.notice) if self.notice else None
         return {
             "kind": self.kind, "id": self.id, "title": self.title,
-            "started": self.started_ts, "elapsed_s": int(time.time() - self.started),
+            # A finished run's clock stops: the page shows the last run
+            # until the next one starts, and a live figure there counts
+            # the time since, not the time it took.
+            "started": self.started_ts,
+            "elapsed_s": (self.finished["duration_s"] if self.finished
+                          else int(time.time() - self.started)),
             "log": lines, "dropped": self.dropped, "prompt": prompt, "notice": notice,
             "finished": self.finished, "aborting": self.aborted.is_set(),
         }