sales: fix another flaky test

tests/codex/sales/states/testsimulatedproving.nim

@@ -58,7 +58,9 @@ asyncchecksuite "sales state 'simulated-proving'":
 
   proc advanceToNextPeriod(market: Market) {.async.} =
     let periodicity = await market.periodicity()
-    clock.advance(periodicity.seconds.truncate(int64))
+    let current = periodicity.periodOf(clock.now().u256)
+    let periodEnd = periodicity.periodEnd(current)
+    clock.set(periodEnd.truncate(int64) + 1)
 
   proc waitForProvingRounds(market: Market, rounds: int) {.async.} =
     var rnds = rounds - 1 # proof round runs prior to advancing