tests/tuples/ttuples_issues.nim: test on all backends + various improvements (#16907)

* improve tests/tuples/ttuples_issues.nim: test on all backends

* address comments

tests/js/readme.md

@@ -0,0 +1,5 @@
+## notes
+Prefer moving tests to a non-js directory so that they get tested across all backends automatically.
+Ideally, tests/js should be reserved to code that only makes sense in js.
+
+Note also that tests for a js specific module (e.g.: `std/jsbigints`) belong to `tests/stdlib`, (e.g.: `tests/stdlib/tjsbigints.nim`)

tests/js/t16822.nim

@@ -1,29 +0,0 @@
-block: # bug #16822
-  var scores: seq[(set[char], int)] = @{{'/'} : 10}
-
-  var x1: set[char]
-  for item in items(scores):
-    x1 = item[0]
-
-  doAssert x1 == {'/'}
-
-  var x2: set[char]
-  for (chars, value) in items(scores):
-    x2 = chars
-
-  doAssert x2 == {'/'}
-
-block: # bug #14574
-  proc fn(): auto =
-    let a = @[("foo", (12, 13))]
-    for (k,v) in a:
-      return (k,v)
-  doAssert fn() == ("foo", (12, 13))
-
-block: # bug #14574
-  iterator fn[T](a:T): lent T = yield a
-  let a = (10, (11,))
-  proc bar(): auto =
-    for (x,y) in fn(a):
-      return (x,y)
-  doAssert bar() == (10, (11,))