Update README with detailed pytest usage

Expanded the testing section to include instructions for running all tests, specific tests, and non-expensive tests using pytest. This provides clearer guidance for contributors on how to execute different test scenarios.

Author: JR-1991

README.md

@@ -204,10 +204,24 @@ export DVUPLOADER_TESTING=true
 
 **3. Run the test(s) with pytest**
 
+Run all tests:
+
 ```bash
 poetry run pytest
 ```
 
+Run a specific test:
+
+```bash
+poetry run pytest -k test_native_upload_with_large_file
+```
+
+Run all non-expensive tests:  
+
+```bash
+poetry run pytest -m "not expensive"
+```
+
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