Need specs for each_with_index + Enumerable method interaction

There are at least three cases from jruby/jruby#8345 where an interaction between an each_with_index Enumerator and an Enumerable method was or is broken in JRuby:

• to_h
• uniq
• zip

Of these, we've fixed uniq and I have a fix in process for to_h but I'm not sure where to put specs for this kind of interaction.

My first thought was in Enumerable specs, since that's where I had to fix things, but I have not come up with a reproduction that doesn't use an each_with_index enumerator. All cases should be specified, though only a few exhibited this differing behavior between CRuby and JRuby.

Running the same example script against TruffleRuby shows similar issues, so I think getting these cases spec'ed would benefit all impls:

$ diff truffleruby_enum_args.txt ruby_enum_args.txt                  
1c1
< truffleruby 24.2.1, like ruby 3.3.7, Oracle GraalVM Native [arm64-darwin20]
---
> ruby 3.3.3 (2024-06-12 revision f1c7b6f435) [arm64-darwin23]
25c25
< filter_map [[1, 2], 0]
---
> filter_map [1, 2]
65c65
< to_h [[1, 2], 0]
---
> to_h [1, 2]
67c67
< uniq [[1, 2], 0]
---
> uniq [1, 2]
70c70
< zip [[[1, 2], 0]]
---
> zip [[1, 2], 0]

It's also possible CRuby is wrong to have special behavior for these methods.

Review the existing Enumerable specs and the issue's example outputs for each_with_index interactions with to_h, uniq, and zip. Reproduce the cases across implementations, then determine where these interactions belong and what behavior the specs should require. Done means all three cases are covered and the expected behavior is resolved.