Docs: Enhance testing documentation with examples and links (#18851)

## Which issue does this PR close?

-  part of #7013 

## Rationale for this change

I was trying to find the right documentation to point people (and AI
tools) at for running the tests and found some improvements to the
existing testing page that would be nice:
https://datafusion.apache.org/contributor-guide/testing.html

## What changes are included in this PR?

Added examples and links for running tests and using test utilities in
the contributor guide.

## Are these changes tested?

I downloaded it locally

## Are there any user-facing changes?

Better testing guide

---------

Co-authored-by: Jeffrey Vo <[email protected]>

Author: alamb

docs/source/contributor-guide/testing.md

@@ -46,19 +46,41 @@ cargo nextest run
 ## Unit tests
 
 Tests for code in an individual module are defined in the same source file with a `test` module, following Rust convention.
-The [test_util](https://github.com/apache/datafusion/tree/main/datafusion/common/src/test_util.rs) module provides useful macros to write unit tests effectively, such as `assert_batches_sorted_eq` and `assert_batches_eq` for RecordBatches and `assert_contains` / `assert_not_contains` which are used extensively in the codebase.
+
+For example, to run tests in the `datafusion` crate:
+
+```shell
+cargo test -p datafusion
+```
+
+The [test_util] module provides useful macros to write unit tests effectively, such as [`assert_batches_sorted_eq`] and [`assert_batches_eq`] for RecordBatches and [`assert_contains`] / [`assert_not_contains`] which are used extensively in the codebase.
+
+[test_util]: https://github.com/apache/datafusion/tree/main/datafusion/common/src/test_util.rs
+[`assert_batches_sorted_eq`]: https://docs.rs/datafusion/latest/datafusion/macro.assert_batches_sorted_eq.html
+[`assert_batches_eq`]: https://docs.rs/datafusion/latest/datafusion/macro.assert_batches_eq.html
+[`assert_contains`]: https://docs.rs/datafusion/latest/datafusion/common/macro.assert_contains.html
+[`assert_not_contains`]: https://docs.rs/datafusion/latest/datafusion/common/macro.assert_not_contains.html
 
 ## sqllogictests Tests
 
-DataFusion's SQL implementation is tested using [sqllogictest](https://github.com/apache/datafusion/tree/main/datafusion/sqllogictest) which are run like other tests using `cargo test --test sqllogictests`.
+DataFusion's SQL implementation is tested using [sqllogictest](https://github.com/apache/datafusion/tree/main/datafusion/sqllogictest). You can run these tests with commands like:
+
+```shell
+# Run all tests
+cargo test --profile=ci --test sqllogictests
+# Run a specific test file
+cargo test --profile=ci --test sqllogictests -- aggregate.slt
+# Run and update expected outputs
+cargo test --profile=ci --test sqllogictests -- --complete
+```
 
-`sqllogictests` tests may be less convenient for new contributors who are familiar with writing `.rs` tests as they require learning another tool. However, `sqllogictest` based tests are much easier to develop and maintain as they 1) do not require a slow recompile/link cycle and 2) can be automatically updated via `cargo test --test sqllogictests -- --complete`.
+`sqllogictests` may be less convenient for new contributors who are familiar with writing `.rs` tests as they require learning another tool. However, `sqllogictest` based tests are much easier to develop and maintain as they 1) do not require a slow recompile/link cycle and 2) can be automatically updated.
 
 Like similar systems such as [DuckDB](https://duckdb.org/dev/testing), DataFusion has chosen to trade off a slightly higher barrier to contribution for longer term maintainability.
 
 DataFusion has integrated [sqlite's test suite](https://sqlite.org/sqllogictest/doc/trunk/about.wiki) as a supplemental test suite that is run whenever a PR is merged into DataFusion. To run it manually please refer to the [README](https://github.com/apache/datafusion/blob/main/datafusion/sqllogictest/README.md#running-tests-sqlite) file for instructions.
 
-## Snapshot testing
+## Snapshot testing (`cargo insta`)
 
 [Insta](https://github.com/mitsuhiko/insta) is used for snapshot testing. Snapshots are generated
 and compared on each test run. If the output changes, tests will fail.
@@ -90,7 +112,7 @@ There are several public interface tests for the DataFusion library in the [test
 You can run these tests individually using `cargo` as normal command such as
 
 ```shell
-cargo test -p datafusion --test parquet_exec
+cargo test -p datafusion --test parquet_integration
 ```
 
 ## SQL "Fuzz" testing