try to fix the flaky test, similarly to the devel version

tests/async/t7758.nim

@@ -1,7 +1,8 @@
 import asyncdispatch
 
 proc task() {.async.} =
-  await sleepAsync(40)
+  const tSleep = 40
+  await sleepAsync(tSleep)
 
 proc main() =
   var counter = 0
@@ -10,6 +11,10 @@ proc main() =
     inc(counter)
     poll(10)
 
-  doAssert counter <= 4
+  const slack = 1
+    # because there is overhead in `async` + `sleepAsync`
+    # as can be seen by increasing `tSleep` from 40 to 49, which increases the number
+    # of failures.
+  doAssert counter <= 4 + slack
 
 for i in 0 .. 4: main()