fix some warning about unused variables in panic tests

[jimexist]

Which issue does this PR close?

Closes #.

Rationale for this change

fix some warning about unused variables in panic tests

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #894 (148487c) into master (2a0a4c3) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #894      +/-   ##
==========================================
- Coverage   82.31%   82.31%   -0.01%     
==========================================
  Files         168      168              
  Lines       48123    48123              
==========================================
- Hits        39614    39613       -1     
- Misses       8509     8510       +1     

Impacted Files	Coverage Δ
arrow/src/array/array_binary.rs	93.07% <100.00%> (ø)
arrow/src/array/array_boolean.rs	94.53% <100.00%> (ø)
arrow/src/array/array_list.rs	95.52% <100.00%> (ø)
arrow/src/array/array_primitive.rs	94.05% <100.00%> (ø)
arrow/src/array/array_string.rs	97.90% <100.00%> (ø)
arrow/src/array/array_struct.rs	89.28% <100.00%> (ø)
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (-0.23%)	⬇️
parquet/src/encodings/encoding.rs	93.52% <0.00%> (-0.20%)	⬇️
arrow/src/datatypes/datatype.rs	65.36% <0.00%> (+0.43%)	⬆️

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