test: Add tests for invocation as a script

All other tests are calling the fixer.main() function (or other internal
functions), but none is really testing the fixer works as a script, and
its arguments work as expected.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

tests/cookiecutter/scripts/test_dependabot-grpc-fixer.py

@@ -3,6 +3,7 @@
 import importlib.util
 import json
 import re
+import runpy
 import shutil
 import sys
 from dataclasses import dataclass
@@ -158,6 +159,64 @@ def test_fixer_cases(
         )
 
 
+def test_fixer_runs_as_script(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """Exercise the module's `__main__` entrypoint."""
+    case = _load_case(CASE_ROOT / "update_runtime_bounds")
+    workspace = _prepare_workspace(case, tmp_path, monkeypatch)
+
+    monkeypatch.setattr(sys, "argv", [str(FIXER_PATH)])
+
+    runpy.run_path(str(FIXER_PATH), run_name="__main__")
+
+    captured = capsys.readouterr()
+    assert (
+        captured.out == "Updated pyproject.toml with 2 grpc/protobuf constraint(s).\n"
+    )
+    assert captured.err == ""
+    assert workspace.read_text(encoding="utf-8") == case.expected_pyproject.read_text(
+        encoding="utf-8"
+    )
+
+
+def test_fixer_runs_as_script_with_custom_pyproject_path(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """Exercise the module's `__main__` entrypoint with a custom path."""
+    case = _load_case(CASE_ROOT / "update_runtime_bounds")
+    workspace = _prepare_workspace(case, tmp_path, monkeypatch)
+    input_pyproject = case.path / "input" / "pyproject.toml"
+    custom_pyproject = tmp_path / "custom" / "pyproject.toml"
+    custom_pyproject.parent.mkdir(parents=True)
+    shutil.copyfile(input_pyproject, custom_pyproject)
+
+    monkeypatch.setattr(
+        sys,
+        "argv",
+        [str(FIXER_PATH), str(custom_pyproject.relative_to(tmp_path))],
+    )
+
+    runpy.run_path(str(FIXER_PATH), run_name="__main__")
+
+    captured = capsys.readouterr()
+    assert (
+        captured.out
+        == "Updated custom/pyproject.toml with 2 grpc/protobuf constraint(s).\n"
+    )
+    assert captured.err == ""
+    assert workspace.read_text(encoding="utf-8") == input_pyproject.read_text(
+        encoding="utf-8"
+    )
+    assert custom_pyproject.read_text(
+        encoding="utf-8"
+    ) == case.expected_pyproject.read_text(encoding="utf-8")
+
+
 @pytest.mark.parametrize(
     ("dependency", "version", "original", "expected"),
     [