TST(string dtype): Resolve xfail in groupby.test_size

[rhshadrach]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

groupby does inference on the group labels across the board.

df = pd.DataFrame({"a": [1, 1, 2], "b": [3, 4, 5]}, dtype="object")
gb = df.groupby("a")
result = gb.sum()
print(result.index.dtype)
# int64

While I agree long-term I'd prefer to preserve object dytpe, I do not think we should be changing this at this point.

[WillAyd]

Why doesn't it work if you just remove the dtype argument and let the constructor infer?

[rhshadrach]

Good question - this was introduced in #55627 but I do not see why if the values are string[pyarrow] that the result would be Int64.

cc @phofl

[jorisvandenbossche]

@rhshadrach the Int64 is for exp_dtype on the line below, not for the dtype of the Index being constructed on this line, so I am not entirely understanding your comment/question ?
(the construction of exp_dtype is not being touched in this PR)