complete future only once in or (fixes #8982) (#9632)

* complete future only once in `or`

Analogous to `and`.
Credits to @k0zmo for proposing the solution.

* add test

tests/async/t8982.nim

@@ -0,0 +1,33 @@
+discard """
+output: '''
+timeout
+runForever should throw ValueError, this is expected
+'''
+"""
+
+
+import asyncdispatch
+
+proc failingAwaitable(p: int) {.async.} =
+  await sleepAsync(500)
+  if p > 0:
+    raise newException(Exception, "my exception")
+
+proc main() {.async.} =
+  let fut = failingAwaitable(1)
+  try:
+    await fut or sleepAsync(100)
+    if fut.finished:
+      echo "finished"
+    else:
+      echo "timeout"
+  except:
+    echo "failed"
+
+
+# Previously this would raise "An attempt was made to complete a Future more than once."
+try:
+  asyncCheck main()
+  runForever()
+except ValueError:
+  echo "runForever should throw ValueError, this is expected"