docs: add instructions for test coverage in integration tests (#967)

This update adds a new section to the troubleshooting documentation
explaining how to collect code coverage from worker subprocesses
when using executorlib. It also adds a link to this new section
in the main README.

The solution involves configuring coverage.py with the subprocess
patch and using 'coverage combine' to merge results from the main
process and workers.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: jan-janssen <3854739+jan-janssen@users.noreply.github.com>

Author: jan-janssen

README.md

@@ -150,6 +150,7 @@ as hierarchical job scheduler within the allocations.
   * [Filesystem Usage](https://executorlib.readthedocs.io/en/latest/trouble_shooting.html#filesystem-usage)
   * [Firewall Issues](https://executorlib.readthedocs.io/en/latest/trouble_shooting.html#firewall-issues)
   * [Message Passing Interface](https://executorlib.readthedocs.io/en/latest/trouble_shooting.html#message-passing-interface)
+  * [Test Coverage for Integration Tests](https://executorlib.readthedocs.io/en/latest/trouble_shooting.html#test-coverage-for-integration-tests)
   * [Python Version](https://executorlib.readthedocs.io/en/latest/trouble_shooting.html#python-version)
   * [Resource Dictionary](https://executorlib.readthedocs.io/en/latest/trouble_shooting.html#resource-dictionary)
   * [SSH Connection](https://executorlib.readthedocs.io/en/latest/trouble_shooting.html#ssh-connection)

docs/trouble_shooting.md

@@ -31,6 +31,25 @@ Typical error messages related to missing dependencies are `ModuleNotFoundError`
 * `ModuleNotFoundError: No module named 'h5py'` - Install [h5py](https://www.h5py.org/) as explained in the [Caching](https://executorlib.readthedocs.io/en/latest/installation.html#caching) section of the installation. 
 * `ModuleNotFoundError: No module named 'networkx'` - Install [networkx](https://networkx.org/) as explained in the [Visualisation](https://executorlib.readthedocs.io/en/latest/installation.html#visualisation) section of the installation.
 
+## Test Coverage for Integration Tests
+When Python functions are executed with executorlib, they run in subprocesses started via `sys.executable`. As a result, `coverage run`
+only tracks the main test process by default and can miss function execution inside executorlib workers.
+
+To collect coverage from both the main process and executorlib subprocesses, enable the subprocess patch in your project configuration:
+```toml
+[tool.coverage.run]
+patch = ["subprocess"]
+```
+
+Then execute:
+```bash
+coverage run -m unittest discover
+coverage combine
+coverage report
+```
+
+The `coverage combine` command merges the data from the main process and subprocesses.
+
 ## Python Version 
 Executorlib supports all current Python version ranging from 3.9 to 3.13. Still some of the dependencies and especially 
 the [flux](http://flux-framework.org) job scheduler are currently limited to Python 3.12 and below. Consequently for high