Follow up on reclassified tests in #18899

[odersky]

#18899 contains three test reclassifications:

• i15922.scala was moved to pending. Not clear whether this should compile, and what changes would be necessary to get it there.
• future-traverse.scala was moved to pending. Not clear how to make this compiler.
• i15749a.scala was moved to neg. The issue description seems to indicate that the test should not compile, but I am not sure what the outcome should be.

We need to follow up on these and either classify them as definite errors / unimplementable or find alternative ways to make them compile.

[Linyxus]

I have reclassified these tests in this commit.

Summary:

• 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.

[Linyxus]

For the limitation mentioned in the previous comment is explained here.