docs: update

docs/developer_guide/testing.md

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+# Testing
+
+After updating the code, ensure that all tests pass. To run the tests, use the following command. 
+
+```bash
+pytest
+```
+
+Once new commits are pushed to the `main` branch, the [`test.yaml`](https://github.com/sinaatalay/rendercv/blob/main/.github/workflows/test.yaml) workflow will be automatically triggered, and the tests will run.
+
+## [`testdata`](https://github.com/sinaatalay/rendercv/tree/main/tests/testdata) folder
+
+In some of the tests:
+
+- RenderCV generates an output with a sample input.
+- Then, the output is compared with a reference output, which has been manually generated and stored in `testdata`. If the files differ, the tests fail.
+
+
+When the `testdata` folder needs to be updated, it can be manually regenerated by setting `update_testdata` to `True` in `conftest.py` and running the tests.
+
+!!! warning
+    - Whenever the `testdata` folder is generated, the files should be reviewed manually to ensure everything works as expected.
+    - `update_testdata` should be set to `False` before committing the changes.
+
+
+