Minor: clean up TODO comments in unnest.slt

[goldmedal]

Which issue does this PR close?

Closes #.

Rationale for this change

What changes are included in this PR?

Remove some legacy TODO comments and refine the tests for unnest.

Are these changes tested?

the original CI

Are there any user-facing changes?

no

[goldmedal]

This issue has been closed. The error assertion makes sense to me. I also added another positive test at L539

[goldmedal]

This error isn't a child expression issue. It's an issue about applying a function to an unnest struct expression. The error message makes sense to me. DuckDB has similar behavior:

D CREATE TABLE unnest_table
  AS VALUES
      ([1,2,3], [7], 1, [13, 14], {'c1': 1, 'c2': 2}),
      ([4,5], [8,9,10], 2, [15, 16], {'c1':3,'c2':4}),
      ([6], [11,12], 3, null, null),
      ([12], [null, 42, null], null, null, {'c1':7,'c2':8}),
      -- null array to verify the `preserve_nulls` option
      (null, null, 4, [17, 18], null)
  ;
D select typeof(unnest(col4)) from unnest_table;
Binder Error: UNNEST() on a struct column can only be applied as the root element of a SELECT expression
LINE 1: select typeof(unnest(col4)) from unnest_table

[goldmedal]

Unnest as a child expression works fine if the input isn't a struct.

[goldmedal]

Although the error message is weird, asserting that this query fails makes sense to me.
DuckDB doesn't allow this SQL, too. I can't find other databases with this behavior. 🤔

D CREATE TABLE unnest_table
  AS VALUES
      ([1,2,3], [7], 1, [13, 14], {'c1': 1, 'c2': 2}),
      ([4,5], [8,9,10], 2, [15, 16], {'c1':3,'c2':4}),
      ([6], [11,12], 3, null, null),
      ([12], [null, 42, null], null, null, {'c1':7,'c2':8}),
      -- null array to verify the `preserve_nulls` option
      (null, null, 4, [17, 18], null)
  ;
D select unnest(col4) u1 from unnest_table group by u1.c1;
Binder Error: Referenced table "u1" not found!
Candidate tables: "unnest_table"
LINE 1: ...st(col4) u1 from unnest_table group by u1.c1;

[goldmedal]

Filed #12794 for it.

[alamb]

Thank you @goldmedal

[alamb]

🚀