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_bad.nim

@@ -0,0 +1,22 @@
+discard """
+  exitCode: 1
+ outputsub: "Error: unhandled exception: foobar: 13"
+"""
+
+# note: this needs `--unhandled-rejections=strict`, see D20210217T215950
+
+import std/asyncjs
+from std/sugar import `=>`
+
+proc fn(n: int): Future[int] {.async.} =
+  if n >= 7: raise newException(ValueError, "foobar: " & $n)
+  else: result = n
+
+proc main() {.async.} =
+  let x1 = await fn(6)
+  doAssert x1 == 6
+  await fn(7).catch((a: Error) => (discard))
+  let x3 = await fn(13)
+  doAssert false # shouldn't go here, should fail before
+
+discard main()