Fix two cases of nondeterminism in voting_schedule.c coverage

After another set of coverage-comparison testing, I found the following to cases of nondeterminism in the tests for voting_schedule.c:

--- a/voting_schedule.c.gcov
+++ b/voting_schedule.c.gcov
@@ -61,7 +61,7 @@
         -:
         1:  next += offset;
         1:  if (next - interval > now)
-    #####:    next -= interval;
+        1:    next -= interval;
         -:
         1:  return next;
         -:}

--- a/voting_schedule.c.gcov
+++ b/voting_schedule.c.gcov
@@ -52,7 +52,7 @@
         -:
         -:  /* Intervals never cross midnight. */
         1:  if (next > midnight_tomorrow)
-    #####:    next = midnight_tomorrow;
+        1:    next = midnight_tomorrow;
         -:
         -:  /* If the interval would only last half as long as it's supposed to, then
         -:   * skip over to the next day. */

I think that these changes are probably dependent on using clock time for our tests, since they all happened around 0:00 UTC.