Update integration testing guide with clean-up step.

Author: QuChen88

docs/development-guide/IntegrationTests.md

@@ -69,3 +69,15 @@ cmd /c ./gradlew --no-parallel --no-daemon test-all-environments
 ```
 
 Test results can be found at `wrapper/build/report/index.html`.
+
+To re-run tests after making code changes, do a clean re-build so that the next test run will pick up the new changes:
+
+macOS:
+```bash
+./gradlew clean
+```
+
+Windows:
+```bash
+cmd /c ./gradlew clean
+```