Add datafusion::test_util, resolve test data paths without env vars

[mluts]

Which issue does this PR close?

Closes #467

Rationale for this change

What changes are included in this PR?

Just ported the code from apache/arrow-rs

Are there any user-facing changes?

Simplified "Examples" section & running tests 🙂

[codecov-commenter]

Codecov Report

Merging #498 (c21f900) into master (01b57f7) will increase coverage by 0.12%.
The diff coverage is 80.88%.

❗ Current head c21f900 differs from pull request most recent head 177cdc0. Consider uploading reports for the commit 177cdc0 to get more accurate results

@@            Coverage Diff             @@
##           master     #498      +/-   ##
==========================================
+ Coverage   75.84%   75.97%   +0.12%     
==========================================
  Files         153      155       +2     
  Lines       25876    26438     +562     
==========================================
+ Hits        19626    20086     +460     
- Misses       6250     6352     +102     

Impacted Files	Coverage Δ
...sta/rust/core/src/serde/logical_plan/from_proto.rs	35.91% <0.00%> (-0.31%)	⬇️
...ta/rust/core/src/serde/physical_plan/from_proto.rs	38.65% <0.00%> (-1.13%)	⬇️
...ista/rust/core/src/serde/physical_plan/to_proto.rs	50.31% <0.00%> (-0.32%)	⬇️
datafusion/src/execution/context.rs	92.08% <ø> (ø)
datafusion/src/optimizer/utils.rs	47.51% <0.00%> (-2.49%)	⬇️
datafusion/src/physical_plan/functions.rs	92.70% <ø> (ø)
python/src/dataframe.rs	0.00% <0.00%> (ø)
python/src/expression.rs	0.00% <0.00%> (ø)
...lista/rust/core/src/serde/logical_plan/to_proto.rs	62.40% <15.38%> (ø)
datafusion/src/optimizer/hash_build_probe_order.rs	58.53% <20.00%> (-1.97%)	⬇️
... and 21 more

---

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[alamb]

Thanks @mluts ! I tried this out locally and it works well.

git clone git@github.com:mluts/arrow-datafusion.git
cd arrow-datafusion/
git checkout  resolve_test_data_paths 
unset PARQUET_TEST_DATA
unset ARROW_TEST_DATA
cargo run --example csv_sql
# failed, but told me to run following command
git submodule update --init
# passes!!
cargo run --example csv_sql

It looks like there are a few linting / test errors, but after that is fixed I think this PR is ready to go:

---- src\datasource\csv.rs - datasource::csv (line 24) stdout ----
error[E0433]: failed to resolve: unresolved import
 --> src\datasource\csv.rs:28:23
  |
7 | let testdata = crate::test_util::arrow_test_data();
  |                       ^^^^^^^^^
  |                       |
  |                       unresolved import
  |                       help: a similar path exists: `datafusion::test_util`

[mluts]

Managed to fix all the checks, yay 🙂

[alamb]

Thanks @mluts looks good!