Re-classify tests

- For i15922, it should not work, as both bad1 and bad2 leak scoped
capabilities. It is moved to neg tests.
- For future-traverse.scala, we could make it work this way. Previously it won't
compile because we instantiated the type parameter of `successful` with a
universal capability, but actually we needn't do that: we could well pick a
local reach capability, as is done in this commit.
- For i15749a.scala, it should work but sadly not right now, due to a limitation
of our current implementation.

Author: Linyxus

tests/neg-custom-args/captures/i15749a.scala

@@ -19,4 +19,4 @@ def test =
   def forceWrapper[A](mx: Wrapper[Unit ->{cap} A]): Wrapper[A] =
     // Γ ⊢ mx: Wrapper[□ {cap} Unit => A]
     // `force` should be typed as ∀(□ {cap} Unit -> A) A, but it can not
-    strictMap[Unit ->{cap} A, A](mx)(t => force[A](t)) // error
+    strictMap[Unit ->{mx*} A, A](mx)(t => force[A](t)) // error // should work

tests/pending/pos-custom-args/captures/i15922.scala renamed to tests/neg-custom-args/captures/i15922.scala

@@ -12,5 +12,5 @@ def leaking(c: Cap^): Id[Cap^{c}] = mkId(c)
 
 def test =
   val ll = (c: Cap^) => leaking(c)
-  val bad1 = withCap(ll)       // used to work, now error (?)
-  val bad2 = withCap(leaking)  // used to work, now error (?)
+  val bad1 = withCap(ll)       // error
+  val bad2 = withCap(leaking)  // error

tests/pending/pos-custom-args/captures/future-traverse.scala renamed to tests/pos-custom-args/captures/future-traverse.scala

@@ -9,7 +9,8 @@ trait Future[+T] { this: Future[T]^ =>
   import Future.*
   def foldLeft[R](r: R): R = r
   def traverse[A, B, M[X] <: IterableOnce[X]](in: M[A]^, bf: BuildFrom[M[A]^, B, M[B]^]): Unit =
-    foldLeft(successful(bf.newBuilder(in)))
+    val builder = bf.newBuilder(in)
+    foldLeft(successful(builder))
 }
 object Future {
   def successful[T](result: T): Future[T] = ???