chore: Add ./scripts/unit-tests.sh (#1515)

# What does this PR do?
Useful for local development. Now you can just trigger the script and
not care about specific arguments to pass to run unit tests.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan

```
$ . ./venv/bin/activate
$ ./scripts/run_tests.sh
$ echo $?
0
```

[//]: # (## Documentation)

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>
Co-authored-by: Nathan Weinberg <31703736+nathan-weinberg@users.noreply.github.com>

.github/workflows/unit-tests.yml

@@ -33,7 +33,7 @@ jobs:
 
       - name: Run unit tests
         run: |
-          uv run --python ${{ matrix.python }} --with-editable . --with-editable ".[dev]" --with-editable ".[unit]" pytest --cov=llama_stack -s -v tests/unit/ --junitxml=pytest-report-${{ matrix.python }}.xml --cov-report=html:htmlcov-${{ matrix.python }}
+          PYTHON_VERSION=${{ matrix.python }} ./scripts/unit-tests.sh --cov=llama_stack --junitxml=pytest-report-${{ matrix.python }}.xml --cov-report=html:htmlcov-${{ matrix.python }}
 
       - name: Upload test results
         if: always()