update docs: when incremental compilation is used, integration tests are preferred (#4336)

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Closes #3567 

## Introduced changes

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-

## Checklist

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- [x] Linked relevant issue
- [x] Updated relevant documentation
- [ ] Added relevant tests
- [ ] Performed self-review of the code
- [ ] Added changes to `CHANGELOG.md`

Author: DelevoXDG

docs/src/testing/running-tests.md

@@ -77,6 +77,13 @@ Tests: 1 passed, 0 failed, 0 ignored, other filtered out
 </details>
 <br>
 
+## Speeding Up Test Compilation
+
+For best cache reuse across repeat runs, prefer the **integration test layout** (`tests/`) over unit tests in `src/`.
+
+Thanks to Scarb's [incremental compilation](https://docs.swmansion.com/scarb/docs/procedural-macros/incremental.html) (enabled by default), the main package and the test code are cached separately, so editing tests does not invalidate the main package's build cache.
+For details on the layout difference, see [Test Collection](test-collection.md).
+
 ## Skipping tests
 
 You can use the `--skip` flag to exclude tests matching a specified filter pattern.

docs/src/testing/test-collection.md

@@ -57,6 +57,14 @@ mod tests {
 
 ### Integration Tests
 
+> 📝 **Note**
+>
+> Thanks to Scarb's [incremental compilation](https://docs.swmansion.com/scarb/docs/procedural-macros/incremental.html) (enabled by default),
+> the integration test layout usually recompiles faster than the unit test layout.
+>
+> The main package and the test code are cached separately, so editing tests does not invalidate the main package's build cache.
+
+
 Integration tests are placed in `tests` directory.
 This directory is a special directory in Scarb.
 Tests do not have to be wrapped in `#[cfg(test)]` and each file is treated as a separate module.