Fix #13889 with testcase (#13896) [backport]

* Fix https://github.com/nim-lang/Nim/issues/13889

* Add testcase

* Reduce test time

Co-authored-by: Elie Zedeck RANDRIAMIANDRIRAY <elie.zedeck@gmail.com>

lib/pure/asyncmacro.nim

@@ -46,7 +46,7 @@ template createCb(retFutureSym, iteratorNameSym,
         else:
           {.gcsafe.}:
             {.push hint[ConvFromXtoItselfNotNeeded]: off.}
-            next.callback = cast[proc() {.closure, gcsafe.}](identName)
+            next.addCallback cast[proc() {.closure, gcsafe.}](identName)
             {.pop.}
     except:
       futureVarCompletions

tests/async/t13889.nim

@@ -0,0 +1,27 @@
+discard """
+  output: '''
+believer Foo is saved:true
+believer Bar is saved:true
+believer Baz is saved:true
+'''
+"""
+
+import asyncdispatch
+
+var
+  promise = newFuture[bool]()
+
+proc believers(name: string) {.async.} =
+  let v = await promise
+  echo "believer " & name & " is saved:" & $v
+
+asyncCheck believers("Foo")
+asyncCheck believers("Bar")
+asyncCheck believers("Baz")
+
+proc savior() {.async.} =
+  await sleepAsync(50)
+  complete(promise, true)
+  await sleepAsync(50) # give enough time to see who was saved
+
+waitFor(savior())