followup #16871 asyncjs.then: allow pipelining procs returning futures (#17189)

* followup #16871 asyncjs.then: allow pipelining procs returning futures
* rename test files where they belong
* fix tests
* tests for then with `onReject` callback
* rename test file containing fail to avoid messing with grep
* address comments
* cleanup
* un-disable 1 test

tests/js/tasyncjs_pragma.nim

@@ -0,0 +1,29 @@
+discard """
+  output: '''
+0
+t
+'''
+"""
+
+# xxx merge into tasyncjs.nim
+
+import asyncjs, macros
+
+macro f*(a: untyped): untyped =
+  assert a.kind == nnkProcDef
+  result = nnkProcDef.newTree(a.name, a[1], a[2], a.params, a.pragma, a[5], nnkStmtList.newTree())
+  let call = quote:
+    echo 0
+  result.body.add(call)
+  for child in a.body:
+    result.body.add(child)
+  #echo result.body.repr
+
+proc t* {.async, f.} =
+  echo "t"
+
+proc t0* {.async.} =
+  await t()
+
+discard t0()
+