Add tests for reading nested decimal arrays from parquet

[tustvold]

Which issue does this PR close?

Related to #2236.

Rationale for this change

We don't really have very good test coverage of decimal types, and whilst reading #2236 I suspected it might break something. This PR adds tests so that we can have confidence that this feature is not being broken even further.

What changes are included in this PR?

Adds some tests

Are there any user-facing changes?

No

[tustvold]

Perhaps unsurprisingly, this is broken. 😅

[alamb]

👍 Testing FTW

[codecov-commenter]

Codecov Report

Merging #2254 (16deaac) into master (3032a52) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2254      +/-   ##
==========================================
+ Coverage   82.29%   82.30%   +0.01%     
==========================================
  Files         243      243              
  Lines       62443    62459      +16     
==========================================
+ Hits        51387    51408      +21     
+ Misses      11056    11051       -5     

Impacted Files	Coverage Δ
parquet/src/arrow/arrow_reader.rs	95.12% <100.00%> (+0.12%)	⬆️
...row/src/array/builder/string_dictionary_builder.rs	90.64% <0.00%> (-0.72%)	⬇️
parquet/src/arrow/schema.rs	96.93% <0.00%> (+0.17%)	⬆️
arrow/src/datatypes/datatype.rs	62.61% <0.00%> (+0.31%)	⬆️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.45%)	⬆️
arrow/src/array/array_decimal.rs	90.96% <0.00%> (+0.64%)	⬆️

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[alamb]

👍 Testing FTW

[alamb]

cc @liukun4515

[ursabot]

Benchmark runs are scheduled for baseline = b879977 and contender = 42b15a8. 42b15a8 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java